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/cmProcessTools.h

@@ -59,7 +59,7 @@ public:
     char Separator;
     char LineEnd;
     bool IgnoreCR;
-    bool ProcessChunk(const char* data, int length) CM_OVERRIDE;
+    bool ProcessChunk(const char* data, int length) override;
 
     /** Implement in a subclass to process one line of input.  It
         should return true only if it is interested in more data.  */
@@ -76,7 +76,7 @@ public:
     }
 
   private:
-    bool ProcessLine() CM_OVERRIDE { return true; }
+    bool ProcessLine() override { return true; }
   };
 
   /** Run a process and send output to given parsers.  */