Protect SetDoubleBuffered against wx3.1

In 3.0, SetDoubleBuffered() is defined only under MSW and Linux.  In
3.1, they change the routine to be a virtual function and so we need to
protect against redefining an existing function (Thanks Ian for pointing
this out!)

include/widgets/wx_menubar.h

@@ -54,9 +54,12 @@ public:
         wxMenuBar::SetAcceleratorTable( wxAcceleratorTable() );
     }
 
+/// In version 3.1, wxWidgets makes SetDoubleBuffered() a virtual function
 #ifdef __WXMAC__
+#if !wxCHECK_VERSION( 3, 1, 0 )
     void SetDoubleBuffered( bool ) { }
 #endif
+#endif
 };
 
 #endif // WX_MENUBAR_H_