Add st_birthtime to struct stat

This is available on OS X, FreeBSD and NetBSD. On Linux, the definition
itself will not result in any errors. However, code that does attempt to
st_birthtime under Linux will now fail at C compile time rather than
Cython compile time.

Cython/Includes/posix/stat.pxd

@@ -18,6 +18,11 @@ cdef extern from "<sys/stat.h>" nogil:
         time_t  st_mtime
         time_t  st_ctime
 
+        # st_birthtime exists on *BSD and OS X.
+        # Under Linux, defining it here does not hurt. Compilation under Linux
+        # will only (and rightfully) fail when attempting to use the field.
+        time_t  st_birthtime
+
 # POSIX prescribes including both <sys/stat.h> and <unistd.h> for these
 cdef extern from "<unistd.h>" nogil:
     int fchmod(int, mode_t)