Remove GNU specific "tm" struct fields "tm_zone" and "tm_gmtoff" from...

Remove GNU specific "tm" struct fields "tm_zone" and "tm_gmtoff" from libc/time.pxd because they get in the way of automatic struct conversions.
See https://github.com/cython/cython/issues/3733

Cython/Includes/libc/time.pxd

@@ -20,8 +20,9 @@ cdef extern from "<time.h>" nogil:
         int  tm_wday
         int  tm_yday
         int  tm_isdst
-        char *tm_zone
-        long tm_gmtoff
+        # GNU specific extensions
+        #char *tm_zone
+        #long tm_gmtoff
 
     int     daylight            # global state
     long    timezone