Various build fixes and improvements

[chewi]

I know you haven't touched this repository in a long time but I would be grateful if you could merge these as it would make maintaining the Gentoo Linux package easier in future.

Just for fun, I cross-compiled it for m68k and ran it under Xfbdev on the ARAnyM emulator. It really works! Sadly my real Amiga is back at the office, which I haven't been to since being locked down. I couldn't get X to work on there anyway. It's probably just too slow.