Turn off warning about deprecated usage of __free_hook.

Turn off warning about deprecated usage of __free_hook.

	Since this is to try and avoid crashes on shutdown, it will either
	be available and work, or linking will fail, which will be detected
	by configure, and thus we will not use it.

	* configure.ac: Add HAVE_FREE_HOOK when __free_hook is available.
	Also fix detection of __after_morecore_hook.
	* src/emacs.c (voodoo_free_hook): Ignore the warning about
	deprecated variable.
	* src/emacs.c (Fkill_emacs): Ditto.

Signed-off-by: Nelson Ferreira <nelson.ferreira@ieee.org>