configure: Define _GNU_SOURCE for Cygwin as well

Cygwin headers are now a bit more correct in handling feature test macros,
so use _GNU_SOURCE when building for Cygwin, as well.

(Notwithstanding f381c27c, we should probably have always been using
_GNU_SOURCE, since asprintf() is used by mesa in places)

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>
Reviewed-by: Jon Turney <jon.turney@dronecode.org.uk>
1 file changed