Provide a `GNUmakefile` and use it to build `miou` - but keep the `dune` file when people vendor `miou`

[dinosaure]

The project is really small and we don't really want to provide much more that we currently have. It can be interesting to finally have a zero-dependency to build and use miou. GNUmakefile (needed for OCaml) can be interesting. We probably shoud have one which produces the same layout that dune does. I know that people from dune work on something which should produce an Makefile from a dune file.

[kit-ty-kate]

From experience with opam-file-format I would personally advise against this.

See ocaml/opam-file-format#51

[dinosaure]

I see. I figured out about this solution because miou does not have a complex way to build (it's just few modules, what I'm worried is more about dune's aliases). The pro will be that miou only depends on what is needed for OCaml which can be nice. But if the maintainance of a GNUmakefile is big, we can just keep the state as is: just use dune.

Let this issue open to see if people are aware about that or not. Thanks for your feedback.