Fix build with wx3.0 under __WXGTK__

While stimfit has been ported to wx3.0, it seems this hasn't been tested
under __WXGTK__ as there doc is always NULL in wxStfApp::CreateUnifiedMenuBar
but we try to call methods on it for wx >= 2.9.0.

I don't understand the reason for all the conditionalisation on __WXGTK__,
__WXMSW__ etc, so this may not be the best fix, but it seems to work.