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>