Centralize all system extensions checks.

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.