MSVC support

[MisterDA]

Hi! It would be nice if we could use the action with the MSVC / clang-cl port, restored in OCaml 5.3. The system-msvc opam package and the ilammy/msvc-dev-cmd action might help.
clang-cl is a drop-in replacement for cl.exe that's also installed in the GitHub workers. It's not clear if it would be easy to come up with a user-facing switch to use it. Maybe users could be allowed to set the CC env var?

[smorimoto]

Thanks for your suggestion! I think adding a few clear examples to the existing EXAMPLES.md document should be sufficient to illustrate this use case. If users can follow a straightforward example or two, it should provide the necessary guidance without complicating things further.

[MisterDA]

I'm not sure examples are sufficient. Something that could help: how would you go about selecting another C compiler to build OCaml with setup-ocaml? Suppose you're on ubuntu-latest, but you'd rather use clang of GCC. How would you pass CC=clang to OCaml's configure script?

[smorimoto]

We've just added MSVC support in #1072 — users can now set windows-compiler: msvc to build OCaml with cl.exe via the system-msvc opam package.

Regarding clang-cl: we initially considered adding it as a third option (setting CC=clang-cl globally), but that approach is rather heavy-handed as it would affect all package builds, not just OCaml itself. The way opam handles MSVC is much cleaner — msvs-detect → conf-msvc64 → ocaml-env-msvc64 keeps the compiler environment properly scoped to the switch.

@dra27 Would it make sense to have something analogous for clang-cl in opam-repository — perhaps a system-clang-cl package or an option package that sets CC=clang-cl within the opam build environment? That way the compiler selection would be handled at the opam level rather than leaking into the global environment. Happy to help with the setup-ocaml side once there's an upstream mechanism in place.