Make it possible to exclude wxDefaultArtProvider from build. Now that a
Tango-based provider exists, it may make sense to disable the standard bitmaps
if the program doesn't need the few of them not provided by Tango anyhow.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66506 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
