fix build when GDK_WINDOWING_X11 is not defined

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78163 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
clarify comment

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78164 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775