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/cmInstallFilesCommand.h

@@ -24,14 +24,14 @@ public:
   /**
    * This is a virtual constructor for the command.
    */
-  cmCommand* Clone() CM_OVERRIDE { return new cmInstallFilesCommand; }
+  cmCommand* Clone() override { return new cmInstallFilesCommand; }
 
   /**
    * 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;
 
   /**
    * This is called at the end after all the information
@@ -39,8 +39,8 @@ public:
    * not implement this method.  At this point, reading and
    * writing to the cache can be done.
    */
-  void FinalPass() CM_OVERRIDE;
-  bool HasFinalPass() const CM_OVERRIDE { return !this->IsFilesForm; }
+  void FinalPass() override;
+  bool HasFinalPass() const override { return !this->IsFilesForm; }
 
 protected:
   void CreateInstallGenerator() const;