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/filesys.cpp

@@ -735,13 +735,13 @@ class wxFileSystemModule : public wxModule
         {
         }
 
-        virtual bool OnInit()
+        virtual bool OnInit() wxOVERRIDE
         {
             m_handler = new wxLocalFSHandler;
             wxFileSystem::AddHandler(m_handler);
             return true;
         }
-        virtual void OnExit()
+        virtual void OnExit() wxOVERRIDE
         {
             delete wxFileSystem::RemoveHandler(m_handler);