gdk: Only use XComposite if it is available

Fixes build if it isn't available. Bug #629748.

gdk/x11/gdkdnd-x11.c

@@ -587,6 +587,7 @@ gdk_window_cache_new (GdkScreen *screen)
 
   g_free (children);
 
+#ifdef HAVE_XCOMPOSITE
   /*
    * Add the composite overlay window to the cache, as this can be a reasonable
    * Xdnd proxy as well.
@@ -600,6 +601,7 @@ gdk_window_cache_new (GdkScreen *screen)
       gdk_window_cache_add (result, cow, 0, 0, gdk_screen_get_width (screen), gdk_screen_get_height (screen), TRUE);
       XCompositeReleaseOverlayWindow (xdisplay, GDK_WINDOW_XWINDOW (root_window));
     }
+#endif
 
   return result;
 }