set: Handle value-less PARENT_SCOPE explicitly

The code "set(VAR PARENT_SCOPE)" unsets VAR in the parent scope.
Implement this case explicitly next to where "set(VAR)" is explicitly
handled to unset VAR in the current scope.

Source/cmSetCommand.cxx

@@ -62,9 +62,17 @@ bool cmSetCommand
     this->Makefile->RemoveDefinition(args[0].c_str());
     return true;
     }
+  // SET (VAR PARENT_SCOPE) // Removes the definition of VAR
+                            // in the parent scope.
+  else if (args.size() == 2 && args[args.size()-1] == "PARENT_SCOPE")
+    {
+    this->Makefile->RaiseScope(variable, 0);
+    return true;
+    }
 
   // here are the remaining options
   //  SET (VAR value )
+  //  SET (VAR value PARENT_SCOPE)
   //  SET (VAR CACHE TYPE "doc String" [FORCE])
   //  SET (VAR value CACHE TYPE "doc string" [FORCE])
   std::string value;  // optional