build: Add feature test macros for glibc 2.19+

glibc introduced _DEFAULT_SOURCE in version 2.19 to replace _BSD_SOURCE and
_SVID_SOURCE, which were deprecated in version 2.20. Add _DEFAULT_SOURCE
where the latter two are used to be forwards-compatible and avoid warnings
about the use of deprecated definitions.

libavformat/os_support.c

@@ -21,6 +21,7 @@
  */
 
 /* needed by inet_aton() */
+#define _DEFAULT_SOURCE
 #define _SVID_SOURCE
 
 #include "config.h"

libavformat/udp.c

@@ -24,6 +24,7 @@
  * UDP protocol
  */
 
+#define _DEFAULT_SOURCE
 #define _BSD_SOURCE     /* Needed for using struct ip_mreq with recent glibc */
 
 #include "avformat.h"

libswscale/utils.c

@@ -20,6 +20,7 @@
 
 #include "config.h"
 
+#define _DEFAULT_SOURCE
 #define _SVID_SOURCE // needed for MAP_ANONYMOUS
 #include <assert.h>
 #include <inttypes.h>