Fix posix_memalign() usage when libavutil is compiled outside FFmpeg.

To use posix_memalign() "#define _XOPEN_SOURCE 600" have to be defined.
FFmpeg configure defines it from the command line through config.mak,
but when libavutil is used outside of FFmpeg the define is missing.
Signed-off-by: Ivan Kalvachev <ikalvachev@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/mem.c

@@ -24,6 +24,8 @@
  * default memory allocator for libavutil
  */
 
+#define _XOPEN_SOURCE 600
+
 #include "config.h"
 
 #include <limits.h>