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/cmLinkLibrariesCommand.h

@@ -25,14 +25,14 @@ public:
   /**
    * This is a virtual constructor for the command.
    */
-  cmCommand* Clone() CM_OVERRIDE { return new cmLinkLibrariesCommand; }
+  cmCommand* Clone() override { return new cmLinkLibrariesCommand; }
 
   /**
    * This is called when the command is first encountered in
    * the CMakeLists.txt file.
    */
   bool InitialPass(std::vector<std::string> const& args,
-                   cmExecutionStatus& status) CM_OVERRIDE;
+                   cmExecutionStatus& status) override;
 };
 
 #endif