HAS_TM_GMTOFF false under Cygwin

Hi,

`struct tm` does not contain a `tm_gmtoff` member under Cygwin. Added clause to `extconf.rb` for that case so macro `HAS_TM_GMTOFF` is false (see line 508 of dump.c).

Tested with an up to date installation of Cygwin. Google suggests Cygwin's `struct tm` has not had `tm_gmtoff` for at least 6 years (and presumably forever) so change should not break older Cygwin users.