Original X11 device demuxer patch from Clemens Fruhwirth

- Build system integration is equivalent to RFC #6 patch
- Same rule applies to ffmpeg.c/allformats.[c|h]
- x11grab.c is from Clemens Fruhwirth except the x11 grab
  structure for libavformat registration is renamed to
  match build system integration.

Originally committed as revision 7293 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -3234,6 +3234,10 @@ static void prepare_grab(void)
 
     if (has_video) {
         AVInputFormat *fmt1;
+#warning FIXME: find a better interface
+        if(!strncmp(video_device,"x11:",4)) {
+            video_grab_format="x11grab";
+        }
         fmt1 = av_find_input_format(video_grab_format);
         vp->device  = video_device;
         vp->channel = video_channel;