Build: make tezos-lmdb compile under FreeBSD (ocaml/opam-repository CI)

Context

Import the patch merged in the github pull request https://github.com/ocaml/opam-repository/pull/17404 that allowed tezos-lmdb to compile there (without breaking compilation under linux/MacOS...)

Not ambitious preliminary step toward !2043 (closed)

Manually testing the MR

Get a FreeBSD, compile there :-)

Checklist

• Provide automatic testing (see the testing guide).

  Good non trivial question to test compilation under BSD in our CI setting... I'm open to suggestion

• Add item in the Development Version section of CHANGES.md (only for new features and bug fixes).

  Is it really worth it?

Reviewers