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/datetime.cpp

@@ -123,14 +123,14 @@ wxCUSTOM_TYPE_INFO(wxDateTime, wxToStringConverter<wxDateTime> , wxFromStringCon
 class wxDateTimeHolidaysModule : public wxModule
 {
 public:
-    virtual bool OnInit()
+    virtual bool OnInit() wxOVERRIDE
     {
         wxDateTimeHolidayAuthority::AddAuthority(new wxDateTimeWorkDays);
 
         return true;
     }
 
-    virtual void OnExit()
+    virtual void OnExit() wxOVERRIDE
     {
         wxDateTimeHolidayAuthority::ClearAllAuthorities();
         wxDateTimeHolidayAuthority::ms_authorities.clear();