Support modular X11 path to rgb.txt patch by (Ville Skyttä - scop from SF-bugtracker)

Originally committed as revision 5429 to svn://svn.ffmpeg.org/ffmpeg/trunk

vhook/imlib2.c

@@ -144,6 +144,8 @@ int Configure(void **ctxp, int argc, char *argv[])
         char buff[256];
         int done = 0;
 
+        f = fopen("/usr/share/X11/rgb.txt", "r");
+        if (!f)
             f = fopen("/usr/lib/X11/rgb.txt", "r");
         if (!f) {
             fprintf(stderr, "Failed to find rgb.txt\n");