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

Back-patch commit 868628e4 into the 9.5 branch, so that we can support
building that branch with Visual Studio 2015.  This patch itself could
go further back, but other VS2015 patches such as 0fb54de9 and c8e81afc
were only back-patched to 9.5, so there seems little point in handling
this one differently.

Discussion: https://postgr.es/m/CAD=LzWFg+Z-KUS3Wm8-1J2vOuYErJXbjuE6b7quzswQEBXJWMQ@mail.gmail.com

Now that we have backported VS2015 support to 9.4 and 9.3, backport this also.