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/cmExtraCodeBlocksGenerator.h

@@ -25,7 +25,7 @@ public:
 
   static cmExternalMakefileProjectGeneratorFactory* GetFactory();
 
-  void Generate() CM_OVERRIDE;
+  void Generate() override;
 
 private:
   struct CbpUnit