ENH: Patch from Alex to document this command as discouraged.

Source/cmVariableRequiresCommand.h

@@ -72,6 +72,12 @@ public:
       "is reported.";
     }
   
+  /** This command is kept for compatibility with older CMake versions. */
+  virtual bool IsDiscouraged()
+    {
+    return true;
+    }
+
   cmTypeMacro(cmVariableRequiresCommand, cmCommand);
 };