Centralize language flag addition for Makefiles

We create cmMakefileTargetGenerator::AddFeatureFlags to consolidate
addition of language flags.  Currently it just adds the flags from
generic per-language flag variables (AddLanguageFlags).

Source/cmMakefileTargetGenerator.h

@@ -220,6 +220,9 @@ protected:
   // Helper to add flag for windows .def file.
   void AddModuleDefinitionFlag(std::string& flags);
 
+  // Add language feature flags.
+  void AddFeatureFlags(std::string& flags, const char* lang);
+
   //==================================================================
   // Convenience routines that do nothing more than forward to
   // implementaitons