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.
Merge pull request #42 from rofl0r/fix_mtim

fix issue with 0S X lacking st_mtim