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/common/artstd.cpp

@@ -35,7 +35,7 @@ class wxDefaultArtProvider : public wxArtProvider
 {
 protected:
     virtual wxBitmap CreateBitmap(const wxArtID& id, const wxArtClient& client,
-                                  const wxSize& size);
+                                  const wxSize& size) wxOVERRIDE;
 };
 
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