Make TOOLKIT variable constant in MSW makefiles.

This fixes duplicate lines defining rules for generic files for more than one
port in the makefiles.

Closes #14979.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74406 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775