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.bcc

@@ -50,6 +50,16 @@ CPP = cpp32 -Sr -oCON
 SHARED = 0
 !endif
 
+# Used toolkit [MSW,GTK]
+!ifndef TOOLKIT
+TOOLKIT = MSW
+!endif
+
+# GTK+ toolkit version [,2]
+!ifndef TOOLKIT_VERSION
+TOOLKIT_VERSION =
+!endif
+
 # Build wxUniversal instead of native port? [0,1]
 !ifndef WXUNIV
 WXUNIV = 0