ffserver: SA_RESTART and unsetenv require _XOPEN_SOURCE

Originally committed as revision 14724 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffserver.c

@@ -19,6 +19,8 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#define _XOPEN_SOURCE 600
+
 #include "config.h"
 #ifndef HAVE_CLOSESOCKET
 #define closesocket close