This change prepares the way for using wxGTK under Windows as this would
still define __WINDOWS__ but use __WXGTK__ instead of __WXMSW__.
Closes#14064.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70796 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775