On all Windows platforms, not just Cygwin, use _timezone and _tzname.

Up until now, we've been using timezone and tzname, but Visual Studio
2015 (for which we wish to add support) no longer declares those
symbols.  All versions since Visual Studio 2003 apparently support the
underscore-equipped names, and we don't support anything older than
Visual Studio 2005, so this should work OK everywhere.  But let's see
what the buildfarm thinks.

Michael Paquier, reviewed by Petr Jelinek