pgsql: On all Windows platforms, not just Cygwin,use _timezone and _tz

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

Back-patch commit 868628e4f 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 0fb54de9a and c8e81afc6
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

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3c0e07a466c97244825abf83f2f14ab94ac57624

Modified Files
--------------
src/include/port.h | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)