Use C++11 override instead of CM_OVERRIDE

We now require C++11 support including `override`.  Drop use of
the old compatibility macro.  Convert references as follows:

    git grep -l CM_OVERRIDE -- '*.h' '*.hxx' '*.cxx' |
      xargs sed -i 's/CM_OVERRIDE/override/g'

Source/cmFunctionCommand.cxx

@@ -18,12 +18,12 @@ public:
   cmFunctionHelperCommand() {}
 
   ///! clean up any memory allocated by the function
-  ~cmFunctionHelperCommand() CM_OVERRIDE {}
+  ~cmFunctionHelperCommand() override {}
 
   /**
    * This is a virtual constructor for the command.
    */
-  cmCommand* Clone() CM_OVERRIDE
+  cmCommand* Clone() override
   {
     cmFunctionHelperCommand* newC = new cmFunctionHelperCommand;
     // we must copy when we clone
@@ -39,10 +39,10 @@ public:
    * the CMakeLists.txt file.
    */
   bool InvokeInitialPass(const std::vector<cmListFileArgument>& args,
-                         cmExecutionStatus&) CM_OVERRIDE;
+                         cmExecutionStatus&) override;
 
   bool InitialPass(std::vector<std::string> const&,
-                   cmExecutionStatus&) CM_OVERRIDE
+                   cmExecutionStatus&) override
   {
     return false;
   }