darwin: allow 64-bit darwin to allocate executable memory

darwin requires _DARWIN_C_SOURCE to be defined for MAP_ANON, which is used by
swscale to determine whether to use malloc() or mmap(). 64-bit darwin does not
have an executable heap, so mmap() must be used instead of malloc(), and
therefore _DARWIN_C_SOURCE must be defined.

Originally committed as revision 31760 to svn://svn.mplayerhq.hu/mplayer/trunk/libswscale