Build fix: .cmxs compilation

[murmour]

Build system fix: native plugin compilation.
The produced .cmxs files work great for my purposes.

[UnixJunkie]

looks like a nice contribution. Why don't you accept it?

[murmour]

Guillaume answered my letter in 2014:

I looked at your pull request. OCaml-R is likely maintained by other people. Look at other repos, and you'll find other committers, with french names. They're the people to deal with until I reestablish a decent working place at home.

[gl-yziquel]

I'm sorry not to have been available for such work.

Life has literally been hell all these years.

I'm really happy pveber took over maintenance of that code.

Thank you all for the interest you had in this piece of code. I has been fun to write.