Define _XOPEN_SOURCE as 600 before including a XSI extension header.

Originally committed as revision 20871 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavdevice/x11grab.c

@@ -35,6 +35,8 @@
  * and Edouard Gomez <ed.gomez@free.fr>.
  */
 
+#define _XOPEN_SOURCE 600
+
 #include "config.h"
 #include "libavformat/avformat.h"
 #include <time.h>