Fix wxDateTime::ParseFormat() when matching TZ offset and in DST

Don't skip MakeFromTimeZone() for the current time zone, this is still
necessary.

Fixes a failure in the unit tests when running during a DST period with
TZ=Europe/London, for example.

Closes https://github.com/wxWidgets/wxWidgets/pull/966

docs/changes.txt

@@ -106,6 +106,7 @@ All:
 - Fix problem with wx-config installation and use under NetBSD (wiz).
 - Avoid spurious errors on thread creation under NetBSD.
 - Improve high DPI support in wxAui (Simon Rozman).
+- Fix a bug with parsing time zones in wxDateTime::ParseFormat() (evileye).
 
 All (GUI):