fix the AC_USE_SYSTEM_EXTENSIONS regression.

[Ralf Wildenhues]

Hi Eric,

OK to simply revert

commit 6add6e92a6136865763edddb9946a8d2469153bb
Author: Eric Blake <address@hidden>
Date:   Tue Sep 11 17:13:13 2007 +0000

    Centralize all system extensions checks.
    * lib/autoconf/specific.m4 (AC_USE_SYSTEM_EXTENSIONS): Inline code
    from AC_AIX, AC_GNU_SOURCE, AC_MINIX.  Add Interix support.
    (AC_AIX, AC_GNU_SOURCE, AC_MINIX): Obsolete, and point to
    AC_USE_SYSTEM_EXTENSIONS.
    (AC_ISC_POSIX): Obsolete, and point to AC_SEARCH_LIBS.
    (AC_XENIX_DIR, AC_IRIX_SUN): Promote proper quoting in AU_DEFUN.
    * doc/autoconf.texi (Posix Variants): Reword this section,
    emphasizing that AC_USE_SYSTEM_EXTENSIONS is the preferred method,
    rather than a series of system-specific checks.
    (Obsolete Macros): Add AC_AIX, AC_GNU_SOURCE, AC_ISC_POSIX,
    AC_MINIX.
    * NEWS: Document this change.
    * THANKS: Update.
    Reported by Martin Koeppe.

Otherwise, do you have any alternative suggestions?

Thanks,
Ralf