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/fmapbase.cpp

@@ -362,7 +362,7 @@ class wxFontMapperModule : public wxModule
 public:
     wxFontMapperModule() : wxModule() { }
 
-    virtual bool OnInit()
+    virtual bool OnInit() wxOVERRIDE
     {
         // a dummy wxFontMapperBase object could have been created during the
         // program startup before wxApp was created, we have to delete it to
@@ -376,7 +376,7 @@ public:
         return true;
     }
 
-    virtual void OnExit()
+    virtual void OnExit() wxOVERRIDE
     {
         wxFontMapperBase::Reset();
     }