libmate-panel-applet: Error if panel-plug is built without X11

Author: wmww

libmate-panel-applet/panel-plug.c

@@ -15,6 +15,12 @@
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <config.h>
+
+#ifndef HAVE_X11
+#error file should only be built when HAVE_X11 is enabled
+#endif
+
 #include <gtk/gtk.h>
 
 #include "panel-plug-private.h"