diff --git a/src/video/x11/SDL_x11modes.c b/src/video/x11/SDL_x11modes.c
index 29307b3a6..b1e3ff478 100644
--- a/src/video/x11/SDL_x11modes.c
+++ b/src/video/x11/SDL_x11modes.c
@@ -18,6 +18,10 @@
      misrepresented as being the original software.
   3. This notice may not be removed or altered from any source distribution.
 */
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
 #include "../../SDL_internal.h"
 
 #if SDL_VIDEO_DRIVER_X11