fix issue with 0S X lacking st_mtim

POSIX 2008+ says that struct stat contains a member st_mtim.
OS X blatantly ignores that.
the struct is called st_mtimespec there, but it gets undefined when
_POSIX_C_SOURCE is set, so we must additionally set _DARWIN_C_SOURCE
to get it back.
(using st_mtime is not correct, as that's a time_t member, not a struct timespec)

http://www.opensource.apple.com/source/xnu/xnu-1456.1.26/bsd/sys/stat.h