This is not a search and replace of __MING32__ with _WIN32.
There are places where __MINGW32__ is still used and all changes are
tested with MSVC 2019 compiler. Although, this commit alone will not work
on MSVC because there are plenty of other things to be done which are
seperated into multiple PR. PR OSGeo#289 is the one that "works" on MSVC and
unix as well. But that contains too many changes which shouldn't in a single
PR. Even though this PR alone won't compile GRASS GIS on MSVC, it sure will
not break existing compilers which I think is very important.

Complete support for MSVC will be ready after 2/3 PRs

OSGeo#289