all: add _DEFAULT_SOURCE locally wherever needed

Glibc 2.20 onwards generates a deprecation warning for usage of _BSD_SOURCE and _SVID_SOURCE.
The solution from man feature_test_macros is to define both _DEFAULT_SOURCE and the old macros.
This solution is on the lines of the one in commit af1818276e.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavformat/img2dec.c

@@ -20,6 +20,7 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#define _DEFAULT_SOURCE
 #define _BSD_SOURCE
 #include <sys/stat.h>
 #include "libavutil/avstring.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
 #define _DARWIN_C_SOURCE // needed for MAP_ANON
 #include <inttypes.h>