Support bytecode only installations

[MisterDA]

I've noticed while hacking on architectures where OCaml only supports bytecode (i386, ppc64, s390x, arm32) that opam-user-setup isn't installable, because it requires ocamlopt. Is there a way around this?

[dra27]

Looks fairly easy to do in Makefile - just needs to decide to build .byte version if the ocamlopt command isn't available.

[dra27]

(or alternatively upgrading it to use Dune, but I don't know if supporting 3.12-4.01 remains a source of pride for @AltGr 🙂)

[MisterDA]

(or alternatively upgrading it to use Dune, but I don't know if supporting 3.12-4.01 remains a source of pride for @AltGr 🙂)

Although the readme reads

A working opam installation and OCaml 4.02.0 or newer are required.

there seem to be some support for earlier versions.