Problem with AC_REQUIRE expansion order

[Andreas Schwab]

When AC_USE_SYSTEM_EXTENSIONS is expanded from a macro that also expands
AC_PROG_CC for the first time then expansion order is wrong:

$ cat configure.ac
AC_INIT
AC_DEFUN([FOO],[
AC_PROG_CC
AC_USE_SYSTEM_EXTENSIONS
])
FOO
AC_OUTPUT
$ ./configure 
checking how to run the C preprocessor... /lib/cpp
checking for grep that handles long lines and -e... /usr/bin/grep
checking for egrep... /usr/bin/grep -E
checking for ANSI C header files... no
checking for sys/types.h... no
checking for sys/stat.h... no
checking for stdlib.h... no
checking for string.h... no
checking for memory.h... no
checking for strings.h... no
checking for inttypes.h... no
checking for stdint.h... no
checking for unistd.h... no
checking for gcc... gcc
checking for C compiler default output file name... a.out
checking whether the C compiler works... yes
checking whether we are cross compiling... no
checking for suffix of executables... 
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
checking for gcc option to accept ISO C89... none needed
checking minix/config.h usability... no
checking minix/config.h presence... no
checking for minix/config.h... no
checking whether it is safe to define __EXTENSIONS__... yes
configure: creating ./config.status

The actual problem is the use of AC_INCLUDES_DEFAULT by
AC_USE_SYSTEM_EXTENSIONS.  It calls
AC_REQUIRE([_AC_INCLUDES_DEFAULT_REQUIREMENTS]), which then pushes the
expansion of _AC_INCLUDES_DEFAULT_REQUIREMENTS before the expansion of
FOO and thus before AC_PROG_CC.

This is also a problem with legacy configure scripts, since AC_AIX and
AC_MINIX are now aliases of AC_USE_SYSTEM_EXTENSIONS.

Andreas.

-- 
Andreas Schwab, SuSE Labs, address@hidden
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."