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/http.cpp

@@ -468,11 +468,11 @@ public:
         m_read_bytes = 0;
     }
 
-    size_t GetSize() const { return m_httpsize; }
+    size_t GetSize() const wxOVERRIDE { return m_httpsize; }
     virtual ~wxHTTPStream(void) { m_http->Abort(); }
 
 protected:
-    size_t OnSysRead(void *buffer, size_t bufsize);
+    size_t OnSysRead(void *buffer, size_t bufsize) wxOVERRIDE;
 
     wxDECLARE_NO_COPY_CLASS(wxHTTPStream);
 };