Add relocatable ocamlbuild.0.16.1, patched to remove a symlink

[ElectreAAS]

Context

In dra27/ocamlbuild#3, I ran a lot of builds on the dependents of ocamlbuild, with and without Steve's patch (link to the specific commit), and to my surprise the patch didn't help in any meaningful way. It seems the problem this commit was supposed to fix was also fixed by David separately.

The second commit on Steve's repo is only useful as a workaround for dune, so it wouldn't make sense to upstream it.

What this PR does

• It moves the target of the latest ocamlbuild to be the official repo + a patch containing the relocatable changes and the symlink patch
• Crucially, it changes the opam file to include the OCAMLBUILD_LIBDIR fix which is present on David's repo

Alternate route

If merging this PR is not seen as a good move, another option is to add my fork of the overlays to the default repositories in the dune code

[ElectreAAS]

cc @Alizter as I can't add you as reviewer for some reason

[Alizter]

As we discussed, is it possible to add separate patch files rather than point to a fork?

[ElectreAAS]

This is done in the latest commit, but please don't merge too quickly, I'm running a test batch to make sure it's working

[ElectreAAS]

I am closing this as no longer needed, since ocaml/dune#13792 is merged and ocaml/dune#9873 is closed, see reasoning there.
In the current state of things, ocamlbuild can be considered relocatable from the point of view of dune