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/cmFindFileCommand.h

@@ -24,7 +24,7 @@ public:
   /**
    * This is a virtual constructor for the command.
    */
-  cmCommand* Clone() CM_OVERRIDE { return new cmFindFileCommand; }
+  cmCommand* Clone() override { return new cmFindFileCommand; }
 };
 
 #endif