Fix building with current git FFmpeg

Fix building with current git FFmpeg

	* m4/sxe-mm.m4 (SXE_MM_CHECK_FFMPEG): Turn off -Werror in the test
	to see if url_fopen() needs ByteIOContext**.

	That stuff is now deprecated in current FFmpeg git repo.  This
	patch allows SXEmacs to build with it, although, with loads of
	warnings.  Yeah, I'm on it. :-)

Signed-off-by: Steve Youngs <steve@sxemacs.org>