replace #ifdef _DEBUG with #ifndef NDEBUG

src/libslic3r/Model.cpp

@@ -1686,7 +1686,7 @@ bool model_volume_list_changed(const ModelObject &model_object_old, const ModelO
     return false;
 }
 
-#ifdef _DEBUG
+#ifndef NDEBUG
 // Verify whether the IDs of Model / ModelObject / ModelVolume / ModelInstance / ModelMaterial are valid and unique.
 void check_model_ids_validity(const Model &model)
 {
@@ -1732,6 +1732,6 @@ void check_model_ids_equal(const Model &model1, const Model &model2)
         }
     }
 }
-#endif /* _DEBUG */
+#endif /* NDEBUG */
 
 }