You shouldn't even have to modify any Makefiles if you build on the M1 hardware itself. But according to reports, genlinetoscr might be crashing on this configuration currently, causing the build to fail:
https://github.com/FrodeSolheim/fs-uae/issues/242.
I haven't looked into it yet. Don't have any M1 hardware, and haven't checked if it is possibly to run a decent virtualized environment of it yet either.