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/unix/threadpsx.cpp

@@ -1777,8 +1777,8 @@ void wxOSXThreadModuleOnExit();
 class wxThreadModule : public wxModule
 {
 public:
-    virtual bool OnInit();
-    virtual void OnExit();
+    virtual bool OnInit() wxOVERRIDE;
+    virtual void OnExit() wxOVERRIDE;
 
 private:
     DECLARE_DYNAMIC_CLASS(wxThreadModule)