configure: fix sys/sysmacros.h check

This patch is a copy of
https://cgit.freedesktop.org/mesa/drm/commit/?id=7040fea0280bad527ed4b3d5eee7d7bfbf303efc
by Adam Jackson.

Commit 43c5a65b03 "configure.ac: use
AC_HEADER_MAJOR to detect major()/minor()" started using AC_HEADER_MAJOR
to detect the header where major() is defined. This caused a regression
on systems where glibc is still providing a deprecated definition of
major() through sys/types.h, leading to a bunch of compiler warnings:

/home/pq/git/weston/libweston/launcher-logind.c: In function ‘launcher_logind_open’:
/home/pq/git/weston/libweston/launcher-logind.c:182:13: warning: In the GNU C Library, "major" is defined
 by <sys/sysmacros.h>. For historical compatibility, it is
 currently defined by <sys/types.h> as well, but we plan to
 remove this soon. To use "major", include <sys/sysmacros.h>
 directly. If you did not intend to use a system-defined macro
 "major", you should undefine it after including <sys/types.h>.
  fd = launcher_logind_take_device(wl, major(st.st_rdev),

The issue has been discussed earlier on
https://lists.gnu.org/archive/html/autoconf/2016-09/msg00013.html

Work around the issue by causing the warning to trigger a build failure
inside AC_HEADER_MAJOR test, so that we get MAJOR_IN_SYSMACROS defined.

Cc: Adam Jackson <ajax@redhat.com>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Sergei Trofimovich <slyfox@gentoo.org>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

configure.ac

@@ -34,7 +34,11 @@ AC_CONFIG_MACRO_DIR([m4])
 
 AC_USE_SYSTEM_EXTENSIONS
 AC_SYS_LARGEFILE
+
+save_CFLAGS="$CFLAGS"
+export CFLAGS="$CFLAGS -Werror"
 AC_HEADER_MAJOR
+CFLAGS="$save_CFLAGS"
 
 AM_INIT_AUTOMAKE([1.11 parallel-tests foreign no-dist-gzip dist-xz color-tests subdir-objects])