update conf-g++ availability

[thatportugueseguy]

This package fails on openbsd:
https://ocaml.ci.dev/github/ahrefs/monorobot/commit/671657f6f27ea6333c077a4a6387ec7ef181c61a/variant/openbsd-76-amd64-4.14_opam-2.3
https://ocaml.ci.dev/github/ahrefs/monorobot/commit/671657f6f27ea6333c077a4a6387ec7ef181c61a/variant/openbsd-76-amd64-5.3_opam-2.3

[raphael-proust]

If ocaml-ci has some openbsd runners, would it make sense for oapm-repo-ci to also have some? (ping @shonfeder what do you think?)

[mtelvers]

Running on 7.6, OpenBSD's pkg_add offers packages gcc and g++ separately. It also includes two versions of g++ so a straight pkg_add g++ returns

Ambiguous: g++ could be g++-11.2.0p15 g++-8.4.0p25

Normally, one would run pkg_add foo-- to avoid this but I can't find an incantation of pkg_add which will install without a prompt. I don't think we should choose a specific version as this will change over time. OpenBSD ships clang++.

[raphael-proust]

@mtelvers so we should have

[ "clang++" ] {os = "openbsd"}

?

And we should probably not have anything for freebsd then?