Use current time, not the Epoch for time zone determination.

When deducing the time zone from struct tm tm_gmtoff field, query tm for the
current time and not the Epoch as the DST rules -- and hence the time zone
offset -- could have changed since 1970. This is notably the case for Ireland
which used a different time zone from 1968 to 1971.

Notice that GetTimeZone() still must return the time zone without taking DST
into account, so we explicitly compensate for DST if it's in effect.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69689 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775