Reintroduce setup for correct initialization in OCaml 4+systhreads

gadmmrequested to merge
gadmm/ocaml-boxroot:systhread_fix into main

We introduce a function boxroot_setup_systhreads to fix the initialization on OCaml 4 + systhreads (see !75 (closed)). Earlier removal of boxroot_setup made it impossible to correctly setup systhreads. Rust users must do by hand what boxroot_setup_systhread does, because this complexifies the build too much otherwise. Replaces !75 (closed).

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