Publish m4 and use it instead conf-m4 for opam packages

[andreypopp]

opam package conf-m4 checks if m4 is available on a system. Instead we can package m4 and use it instead.

https://github.com/esy-ocaml/m4

The problem is that m4 build time is around ~90s~ 53s on my mbp.

[jordwalke]

Hmm. You could lazily build it if it's not available perhaps.