Add wxOVERRIDE and use it in common and wxOSX code.

Make overriding virtual methods more explicit and enable additional checks
provided by C++11 compilers when "override" is used.

Closes #16100.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76173 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/osx/cocoa/statbox.mm

@@ -39,7 +39,7 @@ namespace
         {
         }
 
-        virtual void SetLabel( const wxString& title, wxFontEncoding encoding )
+        virtual void SetLabel( const wxString& title, wxFontEncoding encoding ) wxOVERRIDE
         {
             if (title.empty())
                 [GetNSBox() setTitlePosition:NSNoTitle];