Makefiles: Replace method with Wacom specific API

The existing method uses RelativeRoot NONE and FULL values.  In
principle, those should be segregated interfaces.  Mixing
NONE and FULL into the RelativeRoot enum is a case of

 http://thedailywtf.com/articles/What_Is_Truth_0x3f_

Source/cmLocalUnixMakefileGenerator3.h

@@ -250,8 +250,7 @@ protected:
   void CheckMultipleOutputs(bool verbose);
 
 private:
-  std::string ConvertShellCommand(std::string const& cmd,
-                                  cmOutputConverter::RelativeRoot root);
+  std::string MaybeConvertWatcomShellCommand(std::string const& cmd);
   std::string MakeLauncher(cmCustomCommandGenerator const& ccg,
                            cmGeneratorTarget* target,
                            cmOutputConverter::RelativeRoot relative);