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/cmGlobalVisualStudio7Generator.h

@@ -91,7 +91,7 @@ public:
 
   const char* GetIntelProjectVersion();
 
-  bool FindMakeProgram(cmMakefile* mf) CM_OVERRIDE;
+  bool FindMakeProgram(cmMakefile* mf) override;
 
   /** Is the Microsoft Assembler enabled?  */
   bool IsMasmEnabled() const { return this->MasmEnabled; }