Allow users to choose a C compiler toolchain on windows

[gridbugs]

To effectively use dune as a replacement for opam on windows we might need a way of letting users choose which C compiler to use for compiling native code.

[Leonidas-from-XIV]

Can we detect that from the environment? E.g. if the user has MSVC, use that, if the user has Clang, use that?

[nojb]

Can we detect that from the environment? E.g. if the user has MSVC, use that, if the user has Clang, use that?

Note that on Windows, it is relatively common to have more than one toolchain installed (eg MSVC and Mingw64) so that choosing the right toolchain may require user input.