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/html/helpwnd.cpp

@@ -120,7 +120,7 @@ public:
         SetStandardFonts();
     }
 
-    virtual bool LoadPage(const wxString& location)
+    virtual bool LoadPage(const wxString& location) wxOVERRIDE
     {
         if ( !wxHtmlWindow::LoadPage(location) )
             return false;