MSVC / clang-cl 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?