udp: Define _DARWIN_C_SOURCE

This is required in order to get the IP_MULTICAST_TTL define.

Originally committed as revision 25351 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/udp.c

@@ -25,6 +25,7 @@
  */
 
 #define _BSD_SOURCE     /* Needed for using struct ip_mreq with recent glibc */
+#define _DARWIN_C_SOURCE /* Needed for using IP_MULTICAST_TTL on OS X */
 #include "avformat.h"
 #include <unistd.h>
 #include "internal.h"