config: Add tests for modules-desired features

this adds configure tests for features that modules can take advantage
of -- and if they are not present has reduced or fallback functionality.

	gcc/
	* configure.ac: Add tests for fstatat, sighandler_t, O_CLOEXEC,
	unix-domain and ipv6 sockets.
	* config.in: Rebuilt.
	* configure: Rebuilt.