Fixed bug 3476 - round() needs _GNU_SOURCE on some old systems

Ozkan Sezer

On systems with old glibc, such mine with glibc-2.8, the following warning
is issued and is fixed easily by defining _GNU_SOURCE:

/home/me/SDL2-2.0.5/src/video/x11/SDL_x11modes.c: In function 'CalculateXRandRRefreshRate':
/home/me/SDL2-2.0.5/src/video/x11/SDL_x11modes.c:263: warning: implicit declaration of function 'round'
/home/me/SDL2-2.0.5/src/video/x11/SDL_x11modes.c:263: warning: incompatible implicit declaration of built-in function 'round'

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