Clean up header checks

* configure.ac: Reformat AC_CHECK_HEADERS to keep it sorted and
easily updated, and reduce merging errors in the future.
* system.c: Convert all non-standard #ifdef checks for specific
headers to regular #ifdef HAVE_*_H checks.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
3 files changed