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/cmInstallScriptGenerator.h

@@ -18,10 +18,10 @@ class cmInstallScriptGenerator : public cmInstallGenerator
 public:
   cmInstallScriptGenerator(const char* script, bool code,
                            const char* component, bool exclude_from_all);
-  ~cmInstallScriptGenerator() CM_OVERRIDE;
+  ~cmInstallScriptGenerator() override;
 
 protected:
-  void GenerateScript(std::ostream& os) CM_OVERRIDE;
+  void GenerateScript(std::ostream& os) override;
   std::string Script;
   bool Code;
 };