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/ribbon/page.cpp

@@ -49,7 +49,7 @@ public:
     virtual ~wxRibbonPageScrollButton();
 
 protected:
-    virtual wxBorder GetDefaultBorder() const { return wxBORDER_NONE; }
+    virtual wxBorder GetDefaultBorder() const wxOVERRIDE { return wxBORDER_NONE; }
 
     void OnEraseBackground(wxEraseEvent& evt);
     void OnPaint(wxPaintEvent& evt);