For MSW, check for gdiplus.h availability when using configure but only
support it for MSVC 7+ otherwise. For the other platforms, always support it.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62704 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775