feature request: Add system-cygwin for working with cygwin on Windows

[Alizter]

Like we have for system-mingw and system-msvc, we should have a system-cygwin package so that we can easily setup builds based on Cygwin. This is useful for setup-ocaml and generally having the ability to make sure builds are sound on Cygwin.

[kit-ty-kate]

The system-mingw and system-msvc packages are just normal packages. I'm moving this to opam-repository.

[kit-ty-kate]

cc @dra27

[dra27]

This isn't likely to come from me anytime soon - while I very much like the idea of being able to install Cygwin compilers using a native Windows opam, this is a very real case of cross-compilation.

The existing packages should work perfectly if you use the Cygwin build of opam and normal Unix instructions - it would definitely be nice for setup-ocaml to support that, but the change being made here is more akin to adding, say, FreeBSD support setup-ocaml than tweaking native Windows.

[jmid]

Although not a slick setup-ocaml option, a good step forward would be to get a more recent opam into Cygwin https://cygwin.com/packages/summary/opam.html 🤓