* libc/include/time.h (__TM_GMTOFF): Define on Cygwin.
	(__TM_ZONE): Ditto.
	(struct tm): Add __TM_GMTOFF and __TM_ZONE members if they are defined.
	* libc/include/time.h (__TM_GMTOFF): Remove Cygwin-specific definition.
	(__TM_ZONE): Ditto.