Avoid some -Wundef warning. Check for __WXGTK__ using defined,

not plain #if.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@35218 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/popupcmn.cpp

@@ -259,7 +259,7 @@ void wxPopupTransientWindow::Popup(wxWindow *winFocus)
     // subclass the window which has the focus, and not winFocus passed in or
     // otherwise everything else breaks down
     m_focus = FindFocus();
-#elif __WXGTK__
+#elif defined(__WXGTK__)
     // GTK+ catches the activate events from the popup
     // window, not the focus events from the child window
     m_focus = this;