Error (alert deprecated): Thread.kill

[rwmjones]

From: https://kojipkgs.fedoraproject.org//work/tasks/9111/62899111/build.log

+ dune build -j6
File "src/lib/ounit2/threads/oUnitRunnerThreads.ml", line 147, characters 8-19:
147 |         Thread.kill thread;
              ^^^^^^^^^^^
Error (alert deprecated): Thread.kill
Not implemented, do not use

I suspect this reflects some change in OCaml 4.12, since I cannot reproduce it with 4.11.x.

[rwmjones]

It seems related to this change in the compiler: ocaml/ocaml#9419