I'm sorry, but in the year 2025, this is ridiculous:

$ time ./configure
./configure  13.80s user 12.72s system 69% cpu 38.018 total
$ time make -j48

make  all-recursive
make[1]: Entering directory '/home/tavianator/code/gnu/findutils'
Making all in gl
make[2]: Entering directory '/home/tavianator/code/gnu/findutils/gl'
Making all in lib
make[3]: Entering directory '/home/tavianator/code/gnu/findutils/gl/lib'
  GEN      alloca.h
  GEN      ctype.h
  GEN      endian.h
  GEN      dirent.h
  GEN      fcntl.h
  GEN      error.h
  GEN      float.h
  GEN      malloc/dynarray-skeleton.gl.h
  GEN      malloc/scratch_buffer.gl.h
  GEN      malloc/dynarray.gl.h
  GEN      inttypes.h
  GEN      langinfo.h
  GEN      limits.h
  GEN      locale.h
  GEN      math.h
  GEN      pthread.h
  GEN      sched.h
  GEN      selinux/selinux.h
  GEN      selinux/context.h
  GEN      selinux/label.h
  GEN      stddef.h
  GEN      stdio.h
  GEN      stdlib.h
  GEN      string.h
  GEN      strings.h
  GEN      sys/socket.h
  GEN      sys/stat.h
  GEN      sys/time.h
  GEN      sys/types.h
  GEN      sys/uio.h
  GEN      sys/utsname.h
  GEN      sys/wait.h
  GEN      time.h
  GEN      uchar.h
  GEN      unicase.h
  GEN      unictype.h
  GEN      uninorm.h
  GEN      unistd.h
  GEN      unistr.h
  GEN      unitypes.h
  GEN      uniwidth.h
  GEN      wchar.h
  GEN      wctype.h
make  all-recursive
make[4]: Entering directory '/home/tavianator/code/gnu/findutils/gl/lib'
make[5]: Entering directory '/home/tavianator/code/gnu/findutils/gl/lib'
  CC       libgnulib_a-allocator.o
  CC       libgnulib_a-areadlinkat.o
  CC       libgnulib_a-areadlink.o
  CC       libgnulib_a-argmatch.o
  CC       libgnulib_a-argv-iter.o
  CC       libgnulib_a-basename-lgpl.o
  CC       libgnulib_a-openat-proc.o
  CC       libgnulib_a-bitrotate.o
  CC       libgnulib_a-btoc32.o
  CC       libgnulib_a-btowc.o
  CC       libgnulib_a-c-ctype.o
  CC       libgnulib_a-c-strcasecmp.o
  CC       libgnulib_a-c-strncasecmp.o
  CC       libgnulib_a-c-strcasestr.o
  CC       libgnulib_a-c-strstr.o
  CC       libgnulib_a-c32_apply_type_test.o
  CC       libgnulib_a-c32_get_type_test.o
  CC       libgnulib_a-c32isalnum.o
  CC       libgnulib_a-c32isalpha.o
  CC       libgnulib_a-c32isblank.o
  CC       libgnulib_a-c32iscntrl.o
  CC       libgnulib_a-c32isdigit.o
  CC       libgnulib_a-c32isgraph.o
  CC       libgnulib_a-c32islower.o
  CC       libgnulib_a-c32isprint.o
  CC       libgnulib_a-c32ispunct.o
  CC       libgnulib_a-c32isspace.o
  CC       libgnulib_a-c32isupper.o
  CC       libgnulib_a-c32isxdigit.o
  CC       libgnulib_a-c32tolower.o
  CC       libgnulib_a-c32width.o
  CC       libgnulib_a-canonicalize.o
  CC       libgnulib_a-careadlinkat.o
  CC       libgnulib_a-chdir-long.o
  CC       libgnulib_a-cloexec.o
  CC       libgnulib_a-close-stream.o
  CC       libgnulib_a-closein.o
  CC       libgnulib_a-closeout.o
  CC       libgnulib_a-cycle-check.o
  CC       libgnulib_a-opendir-safer.o
  CC       libgnulib_a-dirname.o
  CC       libgnulib_a-basename.o
  CC       libgnulib_a-dirname-lgpl.o
  CC       libgnulib_a-stripslash.o
  CC       libgnulib_a-endian.o
  CC       libgnulib_a-exitfail.o
  CC       libgnulib_a-fcntl.o
  CC       libgnulib_a-creat-safer.o
  CC       libgnulib_a-open-safer.o
  CC       libgnulib_a-fd-hook.o
  CC       libgnulib_a-fflush.o
  CC       libgnulib_a-file-set.o
  CC       libgnulib_a-filemode.o
  CC       libgnulib_a-filenamecat-lgpl.o
  CC       libgnulib_a-float.o
  CC       libgnulib_a-fopen-safer.o
  CC       libgnulib_a-fpurge.o
  CC       libgnulib_a-freadahead.o
  CC       libgnulib_a-freading.o
  CC       libgnulib_a-fseek.o
  CC       libgnulib_a-fseeko.o
  CC       libgnulib_a-fts.o
  CC       libgnulib_a-getprogname.o
  CC       libgnulib_a-gettime.o
  CC       malloc/libgnulib_a-dynarray_at_failure.o
  CC       malloc/libgnulib_a-dynarray_emplace_enlarge.o
  CC       malloc/libgnulib_a-dynarray_finalize.o
  CC       malloc/libgnulib_a-dynarray_resize.o
  CC       malloc/libgnulib_a-dynarray_resize_clear.o
  CC       malloc/libgnulib_a-scratch_buffer_grow.o
  CC       malloc/libgnulib_a-scratch_buffer_grow_preserve.o
  CC       malloc/libgnulib_a-scratch_buffer_set_array_size.o
  CC       libgnulib_a-hard-locale.o
  CC       libgnulib_a-hash.o
  CC       libgnulib_a-hash-pjw.o
  CC       libgnulib_a-hash-triple-simple.o
  CC       libgnulib_a-human.o
  CC       libgnulib_a-i-ring.o
  CC       libgnulib_a-ialloc.o
  CC       libgnulib_a-idcache.o
  CC       libgnulib_a-localcharset.o
  CC       glthread/libgnulib_a-lock.o
  CC       libgnulib_a-malloca.o
  CC       libgnulib_a-math.o
  CC       libgnulib_a-mbchar.o
  CC       libgnulib_a-mbrtoc32.o
  CC       libgnulib_a-mbrtowc.o
  CC       libgnulib_a-mbscasestr.o
  CC       libgnulib_a-mbslen.o
  CC       libgnulib_a-mbsrtoc32s.o
  CC       libgnulib_a-mbsrtowcs.o
  CC       libgnulib_a-mbsstr.o
  CC       libgnulib_a-mbswidth.o
  CC       libgnulib_a-mbszero.o
  CC       libgnulib_a-mbuiter.o
  CC       libgnulib_a-mbuiterf.o
  CC       libgnulib_a-modechange.o
  CC       libgnulib_a-mountlist.o
  CC       libgnulib_a-nstrftime.o
  CC       glthread/libgnulib_a-once.o
  CC       libgnulib_a-openat-die.o
  CC       libgnulib_a-openat-safer.o
  CC       libgnulib_a-opendirat.o
  CC       libgnulib_a-parse-datetime.o
  CC       libgnulib_a-progname.o
  CC       libgnulib_a-quotearg.o
  CC       libgnulib_a-realloc.o
  CC       libgnulib_a-reallocarray.o
  CC       libgnulib_a-safe-read.o
  CC       libgnulib_a-same-inode.o
  CC       libgnulib_a-save-cwd.o
  CC       libgnulib_a-savedir.o
  CC       libgnulib_a-selinux-at.o
  CC       libgnulib_a-se-context.o
  CC       libgnulib_a-se-label.o
  CC       libgnulib_a-se-selinux.o
  CC       libgnulib_a-setlocale_null.o
  CC       libgnulib_a-setlocale_null-unlocked.o
  CC       libgnulib_a-sockets.o
  CC       libgnulib_a-stat-time.o
  CC       libgnulib_a-stdlib.o
  CC       libgnulib_a-strnlen1.o
  CC       libgnulib_a-sys_socket.o
  CC       glthread/libgnulib_a-threadlib.o
  CC       libgnulib_a-time_rz.o
  CC       libgnulib_a-timespec.o
  CC       unicase/libgnulib_a-tolower.o
  CC       unictype/libgnulib_a-ctype_alnum.o
  CC       unictype/libgnulib_a-ctype_alpha.o
  CC       unictype/libgnulib_a-ctype_blank.o
  CC       unictype/libgnulib_a-ctype_cntrl.o
  CC       unictype/libgnulib_a-ctype_digit.o
  CC       unictype/libgnulib_a-ctype_graph.o
  CC       unictype/libgnulib_a-ctype_lower.o
  CC       unictype/libgnulib_a-ctype_print.o
  CC       unictype/libgnulib_a-ctype_punct.o
  CC       unictype/libgnulib_a-ctype_space.o
  CC       unictype/libgnulib_a-ctype_upper.o
  CC       unictype/libgnulib_a-ctype_xdigit.o
  CC       libgnulib_a-unistd.o
  CC       libgnulib_a-dup-safer.o
  CC       libgnulib_a-fd-safer.o
  CC       libgnulib_a-pipe-safer.o
  CC       unistr/libgnulib_a-u32-chr.o
  CC       unistr/libgnulib_a-u32-cpy.o
  CC       unistr/libgnulib_a-u32-pcpy.o
  CC       unistr/libgnulib_a-u32-strcat.o
  CC       unistr/libgnulib_a-u32-strlen.o
  CC       uniwidth/libgnulib_a-width.o
  CC       libgnulib_a-version-etc.o
  CC       libgnulib_a-version-etc-fsf.o
  CC       libgnulib_a-wctype-h.o
  CC       libgnulib_a-vsnzprintf.o
  CC       libgnulib_a-xmalloc.o
  CC       libgnulib_a-xalloc-die.o
  CC       libgnulib_a-xgetcwd.o
  CC       libgnulib_a-xsize.o
  CC       libgnulib_a-xstrtod.o
  CC       libgnulib_a-xstrtol.o
  CC       libgnulib_a-xstrtoul.o
  CC       libgnulib_a-xstrtol-error.o
  CC       libgnulib_a-xstrtoumax.o
  CC       libgnulib_a-yesno.o
  CC       fopen.o
  CC       asnprintf.o
  CC       mbsrtoc32s-state.o
  CC       mbsrtowcs-state.o
  CC       mktime.o
  CC       printf-args.o
  CC       printf-parse.o
  CC       strerror_r.o
  CC       vasnprintf.o
  AR       libgnulib.a
make[5]: Leaving directory '/home/tavianator/code/gnu/findutils/gl/lib'
make[4]: Leaving directory '/home/tavianator/code/gnu/findutils/gl/lib'
make[3]: Leaving directory '/home/tavianator/code/gnu/findutils/gl/lib'
make[3]: Entering directory '/home/tavianator/code/gnu/findutils/gl'
make[3]: Nothing to be done for 'all-am'.
make[3]: Leaving directory '/home/tavianator/code/gnu/findutils/gl'
make[2]: Leaving directory '/home/tavianator/code/gnu/findutils/gl'
Making all in build-aux
make[2]: Entering directory '/home/tavianator/code/gnu/findutils/build-aux'
make[2]: Nothing to be done for 'all'.
make[2]: Leaving directory '/home/tavianator/code/gnu/findutils/build-aux'
Making all in lib
make[2]: Entering directory '/home/tavianator/code/gnu/findutils/lib'
  CC       buildcmd.o
  CC       dircallback.o
  CC       extendbuf.o
  CC       fdleak.o
  CC       listfile.o
  CC       findutils-version.o
  CC       printquoted.o
  CC       qmark.o
  CC       safe-atoi.o
  CC       splitstring.o
  CC       regextype.o
  CC       bugreports.o
  AR       libfind.a
make[2]: Leaving directory '/home/tavianator/code/gnu/findutils/lib'
Making all in find
make[2]: Entering directory '/home/tavianator/code/gnu/findutils/find'
Making all in .
make[3]: Entering directory '/home/tavianator/code/gnu/findutils/find'
  CC       ftsfind.o
  CC       finddata.o
  CC       fstype.o
  CC       parser.o
  CC       pred.o
  CC       exec.o
  CC       tree.o
  CC       util.o
  CC       sharefile.o
  CC       print.o
  CC       getlimits.o
  AR       libfindtools.a
  CCLD     find
  CCLD     getlimits
make[3]: Leaving directory '/home/tavianator/code/gnu/findutils/find'
Making all in testsuite
make[3]: Entering directory '/home/tavianator/code/gnu/findutils/find/testsuite'
make[3]: Nothing to be done for 'all'.
make[3]: Leaving directory '/home/tavianator/code/gnu/findutils/find/testsuite'
make[2]: Leaving directory '/home/tavianator/code/gnu/findutils/find'
Making all in xargs
make[2]: Entering directory '/home/tavianator/code/gnu/findutils/xargs'
Making all in .
make[3]: Entering directory '/home/tavianator/code/gnu/findutils/xargs'
  CC       xargs.o
  CCLD     xargs
make[3]: Leaving directory '/home/tavianator/code/gnu/findutils/xargs'
Making all in testsuite
make[3]: Entering directory '/home/tavianator/code/gnu/findutils/xargs/testsuite'
make[3]: Nothing to be done for 'all'.
make[3]: Leaving directory '/home/tavianator/code/gnu/findutils/xargs/testsuite'
make[2]: Leaving directory '/home/tavianator/code/gnu/findutils/xargs'
Making all in locate
make[2]: Entering directory '/home/tavianator/code/gnu/findutils/locate'
echo '@set LOCATE_DB /usr/local/var/locatedb' > dblocation.texi.tmp
if test -f dblocation.texi && cmp dblocation.texi.tmp dblocation.texi >/dev/null ; then \
    rm dblocation.texi.tmp ; \
else \
    mv dblocation.texi.tmp dblocation.texi ; \
fi
make  all-recursive
make[3]: Entering directory '/home/tavianator/code/gnu/findutils/locate'
Making all in .
make[4]: Entering directory '/home/tavianator/code/gnu/findutils/locate'
rm -f updatedb
  CC       locate.o
  CC       frcode.o
find=`echo find|sed 's,x,x,'`; \
frcode=`echo frcode|sed 's,x,x,'`; \
copyright=`sed -n '/^# Copyright /{s/^..//;p;q;}' ./updatedb.sh \
  grep .` || exit 1; \
sed \
-e "s,@""bindir""@,/usr/local/bin," \
-e "s,@""libexecdir""@,/usr/local/libexec," \
-e "s,@""LOCATE_DB""@,/usr/local/var/locatedb," \
-e "s,@""VERSION""@,4.10.0.51-1ef8-dirty," \
-e "s,@""PACKAGE_NAME""@,GNU findutils," \
-e "s,@""PACKAGE_BUGREPORT""@,[email protected]," \
-e "s,@""PACKAGE_BUGREPORT_URL""@,https://savannah.gnu.org/bugs/?group=findutils," \
-e "s,@""PACKAGE_URL""@,https://www.gnu.org/software/findutils/," \
-e "s,@""find""@,${find}," \
-e "s,@""frcode""@,${frcode}," \
-e "s,@""SORT""@,/usr/bin/sort," \
-e "s,@""SORT_SUPPORTS_Z""@,true," \
-e "s/@""COPYRIGHT""@/${copyright}/" \
./updatedb.sh > updatedb
  CC       word_io.o
chmod +x updatedb
  CCLD     frcode
  CCLD     locate
make[4]: Leaving directory '/home/tavianator/code/gnu/findutils/locate'
Making all in testsuite
make[4]: Entering directory '/home/tavianator/code/gnu/findutils/locate/testsuite'
make[4]: Nothing to be done for 'all'.
make[4]: Leaving directory '/home/tavianator/code/gnu/findutils/locate/testsuite'
make[3]: Leaving directory '/home/tavianator/code/gnu/findutils/locate'
make[2]: Leaving directory '/home/tavianator/code/gnu/findutils/locate'
Making all in doc
make[2]: Entering directory '/home/tavianator/code/gnu/findutils/doc'
ln -s ../locate/dblocation.texi dblocation.texi
make  all-am
make[3]: Entering directory '/home/tavianator/code/gnu/findutils/doc'
make[3]: Nothing to be done for 'all-am'.
make[3]: Leaving directory '/home/tavianator/code/gnu/findutils/doc'
make[2]: Leaving directory '/home/tavianator/code/gnu/findutils/doc'
Making all in po
make[2]: Entering directory '/home/tavianator/code/gnu/findutils/po'
make[2]: Nothing to be done for 'all'.
make[2]: Leaving directory '/home/tavianator/code/gnu/findutils/po'
Making all in m4
make[2]: Entering directory '/home/tavianator/code/gnu/findutils/m4'
make[2]: Nothing to be done for 'all'.
make[2]: Leaving directory '/home/tavianator/code/gnu/findutils/m4'
Making all in gnulib-tests
make[2]: Entering directory '/home/tavianator/code/gnu/findutils/gnulib-tests'
  GEN      arpa/inet.h
  GEN      sys/ioctl.h
  GEN      signal.h
## ---------------------------------------------------- ##
  GEN      sys/select.h
  GEN      sys/random.h
## ------------------- Gnulib tests ------------------- ##
## You can ignore compiler warnings in this directory.  ##
## ---------------------------------------------------- ##
make  all-recursive
make[3]: Entering directory '/home/tavianator/code/gnu/findutils/gnulib-tests'
Making all in .
make[4]: Entering directory '/home/tavianator/code/gnu/findutils/gnulib-tests'
  CC       locale.o
  CC       arpa_inet.o
  CC       c32tob.o
  CC       binary-io.o
  CC       concat-filename.o
  CC       dtotimespec.o
  CC       fd-safer-flag.o
  CC       imaxtostr.o
  CC       dup-safer-flag.o
  CC       inttostr.o
  CC       offtostr.o
  CC       umaxtostr.o
  CC       uinttostr.o
  CC       ioctl.o
  CC       localename.o
  CC       localename-unsafe.o
  CC       localename-table.o
  CC       nanosleep.o
  CC       priv-set.o
  CC       pthread-rwlock.o
  CC       tempname.o
  CC       glthread/thread.o
  CC       time.o
  CC       timespec-add.o
  CC       timespec-sub.o
  CC       tmpdir.o
  CC       unistr/u32-set.o
  CC       unlinkdir.o
  CC       xconcat-filename.o
  CC       test-localcharset.o
  AR       libtests.a
  CCLD     current-locale
  CCLD     test-localcharset
make[4]: Leaving directory '/home/tavianator/code/gnu/findutils/gnulib-tests'
make[3]: Leaving directory '/home/tavianator/code/gnu/findutils/gnulib-tests'
make[2]: Leaving directory '/home/tavianator/code/gnu/findutils/gnulib-tests'
make[2]: Entering directory '/home/tavianator/code/gnu/findutils'
make[2]: Leaving directory '/home/tavianator/code/gnu/findutils'
make[1]: Leaving directory '/home/tavianator/code/gnu/findutils'
make -j48  12.05s user 4.70s system 593% cpu 2.822 total

I paid good money for my 24 CPU cores, but ./configure can only manage to use 69% of one of them. As a result, this random project takes about 13.5× longer to configure the build than it does to actually do the build.

The purpose of a ./configure script is basically to run the compiler a bunch of times and check which runs succeeded. In this way it can test whether particular headers, functions, struct fields, etc. exist, which lets people write portable software. This is an embarrassingly parallel problem, but Autoconf can't parallelize it, and neither can CMake, neither can Meson, etc., etc.

The problem is that most build configurations scripts pretty much look like this:

CFLAGS="-g"
if $CC $CFLAGS -Wall empty.c; then
    CFLAGS="$CFLAGS -Wall"
fi
...

: >config.h
if $CC $CFLAGS have_statx.c; then
    echo "#define HAVE_STATX 1" >>config.h
else
    echo "#define HAVE_STATX 0" >>config.h
fi
...

This is written in an inherently sequential way, but in principle many of these tests could be run in parallel. In fact, we already have an effective tool for parallelizing lots of commands (make), so let's use it. We'll have a configuration makefile that generates our Makefile and config.h:

# The default goal generates both outputs, and merges the logs together
config: Makefile config.h
	cat Makefile.log config.h.log >[email protected]
	rm Makefile.log config.h.log

To start with, we'll save the initial values of variables like CC and CFLAGS into the Makefile:

# Default values, if unspecified
CC ?= cc
CPPFLAGS ?= -D_GNU_SOURCE
CFLAGS ?= -g
LDFLAGS ?=

# Export these through the environment to avoid stripping backslashes
export _CC=${CC}
export _CPPFLAGS=${CPPFLAGS}
export _CFLAGS=${CFLAGS}
export _LDFLAGS=${LDFLAGS}

Makefile:
	printf 'CC := %s\n' "$$_CC" >$@
	printf 'CPPFLAGS := %s\n' "$$_CPPFLAGS" >>$@
	printf 'CFLAGS := %s\n' "$$_CFLAGS" >>$@
	printf 'LDFLAGS := %s\n' "$$_LDFLAGS" >>$@

Using export like this avoids stripping the necessary backslashes from invocations like

$ ./configure CPPFLAGS='-DMACRO=\"string\"'

Now let's check which flags our compiler supports. We'll use this helper script:

#!/bin/sh

set -eu

VAR="$1"
FLAGS="$2"
shift 2

if "$@" $FLAGS; then
    printf '%s += %s\n' "$VAR" "$FLAGS"
fi

When we run

$ ./flags.sh CFLAGS -Wall cc empty.c

it will print

CFLAGS += -Wall

if cc empty.c -Wall succeeds (and nothing otherwise). We can use this to generate some makefile fragments that enable only the supported flags.

ALL_FLAGS = ${CPPFLAGS} ${CFLAGS} ${LDFLAGS}

# Run the compiler with the given flags, sending
#
# - stdout to foo.mk (e.g. CFLAGS += -flag)
# - stderr to foo.mk.log (e.g. error: unrecognized command-line option ‘-flag’)
# - the compiled binary to foo.mk.out
#   - but then we delete it immediately
TRY_CC = ${CC} ${ALL_FLAGS} empty.c -o [email protected] >$@ 2>[email protected] && rm -f [email protected] [email protected]

deps.mk:
	./flags.sh CPPFLAGS "-MP -MD" ${TRY_CC}
Wall.mk:
	./flags.sh CFLAGS -Wall ${TRY_CC}
pthread.mk:
	./flags.sh CFLAGS -pthread ${TRY_CC}
bind-now.mk:
	./flags.sh LDFLAGS -Wl,-z,now ${TRY_CC}

Each of these targets generates a tiny makefile fragment that's responsible for a single flag. Importantly, each one can run independently, in parallel. Once they're done, we can merge them all into the main Makefile and clean up the cruft:

FLAGS := \
    deps.mk \
    Wall.mk \
    pthread.mk \
    bind-now.mk

Makefile: ${FLAGS}
	printf 'CC := %s\n' "$$_CC" >$@
	...
	cat ${FLAGS} >>$@
	cat ${FLAGS:%=%.log} >[email protected]
	rm ${FLAGS} ${FLAGS:%=%.log}

The last part to add to the Makefile is the part that actually builds our application. We can write a simple makefile like this:

OBJS := main.o

app: ${OBJS}
	${CC} ${CFLAGS} ${LDFLAGS} ${OBJS} -o $@

${OBJS}:
	${CC} ${CPPFLAGS} ${CFLAGS} -c ${@:.o=.c} -o $@

-include ${OBJS:.o=.d}

And append it to the Makefile after all the flags:

We also want to generate a config.h file, which defines macros that tell us whether certain libraries/headers/functions/struct fields/etc. exist. We can do this by test-compiling some simple C programs. As an example, these programs check for the various ways to learn about a file's creation timestamp:

#!/bin/sh

set -eu

MACRO=$1
shift

if "$@"; then
    printf '#define %s 1\n' "$MACRO"
else
    printf '#define %s 0\n' "$MACRO"
fi

will output things like

#define HAVE_STATX 1

or

#define HAVE_ST_BIRTHTIM 0

depending on whether the build succeeds. We can use it in a makefile like this:

# Use a recursive make to pick up our auto-detected *FLAGS from above
config.h: Makefile
	+${MAKE} -f header.mk $@

# Get the final *FLAGS values from the Makefile
include Makefile

# We first generate a lot of small headers, before merging them into one big one
HEADERS := \
    have_statx.h \
    have_st_birthtim.h \
    have_st_birthtimespec.h \
    have___st_birthtim.h

# Strip .h and capitalize the macro name
MACRO = $$(printf '%s' ${@:.h=} | tr 'a-z' 'A-Z')

ALL_FLAGS = ${CPPFLAGS} ${CFLAGS} ${LDFLAGS}

${HEADERS}:
	./define.sh ${MACRO} ${CC} ${ALL_FLAGS} ${@:.h=.c} -o [email protected] >$@ 2>[email protected]
	rm -f [email protected] [email protected]

And to join them all together (along with a header guard):

config.h: ${HEADERS}
	printf '#ifndef CONFIG_H\n' >$@
	printf '#define CONFIG_H\n' >>$@
	cat ${HEADERS} >>$@
	printf '#endif\n' >>$@
	cat ${HEADERS:%=%.log} >[email protected]
	rm ${HEADERS} ${HEADERS:%=%.log}

The last step is to wrap configure.mk in a shell script, so people can run ./configure like they're used to:

#!/bin/sh

set -eu

# Guess a good number for make -j<N>
jobs() {
    {
        nproc \
            || sysctl -n hw.ncpu \
            || getconf _NPROCESSORS_ONLN \
            || echo 1
    } 2>/dev/null
}

# Default to MAKE=make
MAKE="${MAKE-make}"

# Set MAKEFLAGS to -j$(jobs) if it's unset
export MAKEFLAGS="${MAKEFLAGS--j$(jobs)}"

$MAKE -r -f configure.mk "$@"

I put together a simple proof-of-concept GitHub repository that contains the full version of all these files if you want to copy-paste. The demo app prints file creation times, if it can figure out how to on your platform.

$ time ./configure
./configure  1.44s user 1.78s system 802% cpu 0.401 total
tavianator@tachyon $ time make -j48
make -j48  1.89s user 0.64s system 817% cpu 0.310 total