Modernize AC_INCLUDES_DEFAULT and friends.

Modernize AC_INCLUDES_DEFAULT and friends.

 * lib/autoconf/headers.m4 (_AC_INCLUDES_DEFAULT_REQUIREMENTS):
   Include stddef.h, stdlib.h, and string.h unconditionally.
   Don't include memory.h at all.
   Don't use AC_HEADER_STDC.
   Don't check for stddef.h, stdlib.h, string.h, or memory.h.
   For compatibility, unconditionally define STDC_HEADERS,
   HAVE_STDLIB_H, and HAVE_STRING_H.
   (AN_HEADER list): Remove C89 headers, and memory.h from list.
   (AC_HEADER_STDC, AC_UNISTD_H): AU_DEFUN to trigger
   _AC_INCLUDES_DEFAULT_REQUIREMENTS if it hasn't already happened,
   and do nothing else.
   (AC_HEADER_TIME): AU_DEFUN, and define TIME_WITH_SYS_TIME unconditionally
   as long as sys/time.h is present.
   (AC_USG, AC_MEMORY_H): Assume existence of string.h.
 * lib/autoconf/functions.m4 (_AC_FUNC_MALLOC_IF, _AC_FUNC_REALLOC_IF):
   Don't use AC_HEADER_STDC. Assume stdlib.h exists.
   (AC_FUNC_MKTIME): Don't use AC_HEADER_TIME.  Assume time.h exists.
   (AC_FUNC_ALLOCA): Assume stdlib.h exists.
   (_AC_LIBOBJ_FNMATCH): Assume wchar.h and wctype.h exist.
   (_AC_LIBOBJ_GETLOADAVG): Assume locale.h exists.
   (AC_FUNC_MMAP): Assume stdlib.h exists.
 * tests/tools.at: Use AC_WORDS_BIGENDIAN instead of AC_STDC_HEADERS in
   autoupdate test.

 * NEWS, doc/autoconf.texi: Document changes. Remove obsolete advice.