configure.ac: Enable AC_USE_SYSTEM_EXTENSIONS

AC_USE_SYSTEM_EXTENSIONS enables _XOPEN_SOURCE, _GNU_SOURCE and similar
macros to expose the largest extent of functionality supported by the
underlying system.  This is required since these macros are often
limiting rather than merely additive, e.g. _XOPEN_SOURCE will actually
on some systems hide declarations which are not part of the X/Open spec.

Since this goes into config.h rather than the command line, ensure all
source is consistently including config.h before anything else,
including system libraries.  This doesn't need to be guarded by a
HAVE_CONFIG_H ifdef, which was only ever a hangover from the X.Org
modular transition.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>

[pq: rebased and converted more files]