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/generic/vscroll.cpp

@@ -49,7 +49,7 @@ public:
         m_scrollHelper = scrollHelper;
     }
 
-    virtual bool ProcessEvent(wxEvent& event);
+    virtual bool ProcessEvent(wxEvent& event) wxOVERRIDE;
 
 private:
     wxVarScrollHelperBase *m_scrollHelper;