diff --git a/gtk/fnmatch.c b/gtk/fnmatch.c
index 04eced312f..f92beb6f1b 100644
--- a/gtk/fnmatch.c
+++ b/gtk/fnmatch.c
@@ -20,10 +20,14 @@ Cambridge, MA 02139, USA.  */
 #endif
 
 #include <errno.h>
+
 /* Added for GTK. We need to make sure that all constants are defined
  * to properly compile this file */
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 #include "fnmatch.h"
+
 #include <ctype.h>