Add PREFIX support to makefile, and distro install command

[alichtman]

Addresses some discussion in #48

[motiejus]

• We don't need a distro-specific target, because distros set PREFIX, and install-for-distro is nonstandard, which defeats the purpose. :)
• use install instead of cp: https://www.man7.org/linux/man-pages/man1/install.1.html

I will also test this on NixOS when I get some time.

[motiejus]

I will also test this on NixOS when I get some time.

Works: NixOS/nixpkgs#323471