Check Xfixes.h header presence before enabling x11grab.

This fixes compilation when lib is present but not header.

Originally committed as revision 20673 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2476,6 +2476,7 @@ enabled jack_indev && check_lib2 jack/jack.h jack_client_open -ljack
 enabled x11grab                         &&
 check_header X11/Xlib.h                 &&
 check_header X11/extensions/XShm.h      &&
+check_header X11/extensions/Xfixes.h    &&
 check_func XOpenDisplay -lX11           &&
 check_func XShmCreateImage -lX11 -lXext &&
 check_func XFixesGetCursorImage -lX11 -lXext -lXfixes