__MINGW64_TOOLCHAIN__ macro is more readable and shorter than the standard
predefined __MINGW64_VERSION_MAJOR and __MINGW32_TOOLCHAIN__ is defined for
the symmetry and also because it will make many tests simpler as we often
need to test not so much for MinGW-w64 for its own sake but rather to disable
the workarounds for MinGW32 when using it.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74885 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
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