Remove symbol WIN32_ONLY_COMPILER

Magnus Hagander <magnus@hagander.net>

Commit: a4777f35565b80ae10605d6d417e5d173156f7da
Author: Magnus Hagander <magnus@hagander.net>
Date: 2017-04-11T13:22:21Z
Releases: 10.0
Remove symbol WIN32_ONLY_COMPILER

This used to mean "Visual C++ except in those parts where Borland C++
was supported where it meant one of those". Now that we don't support
Borland C++ anymore, simplify by using _MSC_VER which is the normal way
to detect Visual C++.

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