Allow building wxGTK under Windows with MSVC.

Replace a few more __WXMSW__ tests with __WINDOWS__ ones and modify bakefiles
to allow specifying the toolkit to be built in wxMSW makefiles.

Closes #13902.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73290 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

build/msw/config.gcc

@@ -40,6 +40,12 @@ CPP ?= $(CC) -E
 # What type of library to build? [0,1]
 SHARED ?= 0
 
+# Used toolkit [MSW,GTK]
+TOOLKIT ?= MSW
+
+# GTK+ toolkit version [,2]
+TOOLKIT_VERSION ?=
+
 # Build wxUniversal instead of native port? [0,1]
 WXUNIV ?= 0