wx/setup.h can't use wxCHECK_VISUALC_VERSION() which is defined in
wx/platform.h _after_ wx/setup.h inclusion.
Replace wxCHECK_VISUALC_VERSION() with manual tests for _MSC_VER.
Also add more comments to explain what's going on.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63842 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775