The easiest "tool" is to build as early as possible against both GNUstep and macOS (iOS etc.), preferably with a good suite of unit tests. Works perfectly fine

The easiest "tool" is to build as early as possible against both GNUstep and macOS (iOS etc.), preferably with a good suite of unit tests. Works perfectly fine, and I don't see how some external tool attempting to give a guess at this would be easier, never mind as accurate.