This is a long-overdue update, so there are many changes. Details are in
the Changes file in the distribution.
The mk/ocaml file will be moved to lang/ocaml and the logic for selecting
whether to use the native-code compiler will be factored out into
native.mk to avoid duplications.
This is the final version before OCaml 5.0 arrives, which has
multicore support and should be a major change from the 4.x versions.
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