* lib/fcntl_.h: Prefer #include_next <foo.h> to #include

* lib/fcntl_.h: Prefer #include_next <foo.h> to #include
@ABSOLUTE_FOO_H@ if @HAVE_INCLUDE_NEXT@.  This works better with
GCC 4.2, which otherwise issues a lot of warnings.
* lib/inttypes_.h, lib/math_.h, lib/search_.h, lib/signal_.h:
* lib/stdint_.h, lib/stdio_.h, lib/stdlib_.h, lib/string_.h:
* lib/sys_time_.h, lib/time_.h, lib/wchar_.h, lib/wctype_.h: Likewise.
* lib/stdlib_.h: Don't bother with #pragma GCC system_header, as
it should no longer be needed.
* lib/string_.h: Likewise.
* modules/absolute-header (HAVE_INCLUDE_NEXT): New 'make' define.
* modules/fcntl (fcntl.h): Substitute @HAVE_INCLUDE_NEXT@.
* modules/inttypes (inttypes.h): Likewise.
* modules/math (math.h): Likewise.
* modules/search (search.h): Likewise.
* modules/signal (signal.h): Likewise.
* modules/stdint (stdint.h): Likewise.
* modules/stdio (stdio.h): Likewise.
* modules/stdlib (stdlib.h): Likewise.
* modules/string (string.h): Likewise.
* modules/sys_time (sys/time.h): Likewise.
* modules/time (time.h): Likewise.
* modules/wchar (wchar.h): Likewise.
* modules/wctype (wtype.h): Likewise.