Fix VC++ warnings about __has_include().

The compiler warns about content after defined(__has_included), so avoid
the issue by putting these Clang tests inside one big #ifdef
__has_included block.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74461 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775