relocatable ocamlbuild

[shonfeder]

There's a PR to address this in dra27/ocamlbuild#3

[Alizter]

The idea here is to pick the reloctable version of ocamlbuild. Once we agree that it is working well and all the patches we have proposed make sense we can make some effort to upstream them. The maintainers there are receptive to such well-motivated changes.

[Alizter]

• overlay patch Add relocatable ocamlbuild.0.16.1, patched to remove a symlink ocaml-dune/opam-overlays#10