Add support for OCaml 4.12

[kit-ty-kate]

mkdir and rmdir were added in ocaml/ocaml#9797

[hannesm]

thanks for your patch, I approve the first commit (add mkdir & rmdir), but would for now delay the second commit until opam 2.1 semantics is settled (in respect to opam var).

[kit-ty-kate]

My reasoning behind adding --readonly is that it is always correct to have it anyway so whether or not opam 2.1 is broken (which I would argue it is, I agree). The option itself was added in opam 2.0.0.
However if you really don't want it and want to make a new release quickly I'll remove it no problem. I'll keep it in opam-alpha-repository though as I need it for testing.

[hannesm]

@kit-ty-kate my reasoning is "we use at 10 other places opam config var prefix, so if we add --readonly here, let's go the full path down and use it everywhere". it does not hurt to add it here, since opam 2.0 supports it already, as you mentioned. but it hurts a bit to have it not consistent across the packages I care about.

[kit-ty-kate]

Fair enough, removed.

[mato]

5476e34 looks good. You are still missing an update to the opam metadata, and we should do something about CI; it's still only testing with OCaml up to 4.10 (i.e. missing 4.11 and 4.12, but the latter presumably won't work until actually released?)

[hannesm]

I rebased and force-pushed. The issue with a CI for OCaml 4.12 is that we need the ocaml-src package in opam-repository first (which is debatable whether we want to have this for pre-releases).

[kit-ty-kate]

This PR is good to go

[hannesm]

The CI issue is (same as observed in mirage/mirage-tcpip#439 (comment)):

ld: /home/opam/.opam/4.08/lib/pkgconfig/../../lib/ocaml-freestanding/libasmrun.a(startup_aux_n.o):/home/opam/.opam/4.08/.opam-switch/build/ocaml-freestanding.0.6.3/ocaml/runtime/startup_aux.c:35: multiple definition of `caml_atom_table'; /home/opam/.opam/4.08/lib/pkgconfig/../../lib/ocaml-freestanding/libasmrun.a(startup_nat_n.o):/home/opam/.opam/4.08/.opam-switch/build/ocaml-freestanding.0.6.3/ocaml/runtime/startup_nat.c:47: first defined here

Interestingly, this only happens on GNU/Linux (with 4.08.1), but not on FreeBSD (see the cirrus CI run). To move forward (and not waste several days investigating build systems), I suggest to drop 4.08 support from freestanding (and MirageOS). Any objections? /cc @mato