Like. He's done the first 90%, leaving only the other 90%. And I mean that a little as a joke but also very sincerely. The supported platforms are a decent starting point, but mainstream OSs on little-endian 64-bit processors doesn't strike me as "the hard targets". NetBSD ( https://wiki.netbsd.org/ports/ ) is ultra portable. This is just portable.

Ultra might be a step below total, but the 3 most common OSs on the 2 most common ISAs is ... several steps below ultra.