Add the warning flag to the C/C++ compiler invocation

[MisterDA]

See Sets of warnings for foreign stubs #8597.

There are some preliminary commits around naming in the C/C++ compiler handling code, to align the terminology with the upstream OCaml compiler, then a few fixes, then the gist of it.

I add -Wall to the C/C++ compiler invocation if GCC/clang are detected, and -W2 if MSVC or clang-cl are detected. This should help C/C++ OCaml FFI writers to set warnings on by default, which they tend to forget, and that's a huge mistake. It is hard to add the right set of warnings in Dune files, see below. If they simply add -Wall, they forget about MSVC and clang-cl. Users usually don't know or don't care about these toolchains. Bugs, typos, anything can creep up. Some packages use a discover script with dune-configurator. This is another possible way of forgetting warnings. I believe we should help users catching problems early on by turning on the C/C++ warnings for them.

Note that as long as we don't set the warning as error flag (-Werror on cc or -WX for msvc), compilation can't fail. No workflow should be broken, the user will just see new warnings reported by the compiler. Users should still be able to disable these flags with :standard \ -Wall.

(rule
 (with-stdout-to c_flags.cc (echo "(-Wall)")))

(rule
 (with-stdout-to c_flags.msvc (echo "(/W2)")))

(library
 (name mylib)
 (foreign_stubs
  (language c)
  (flags :standard (:include c_flags.%{ocaml-config:ccomp_type}))
  (names mystubs)))

This PR is best reviewed commit by commit.

cc @voodoos @rgrinberg @nojb

[nojb]

I am roughly in favour and I agree that more warnings is better than less warnings. However, certain users will probably be annoyed by the change (I am thinking of certain warnings that are a bit complicated to "fix", eg alloc-size-larger-than; see eg https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85783#c3).

I will try to take a look at the patch later in the week.