Dune: add `-safe-string` to default flags

[c-cube]

With 4.06 out, it's reasonable to have this as a default flag.

[jeremiedimino]

This would only change the behaviour for OCaml < 4.06, is that correct?

[kit-ty-kate]

Yes, and for 4.06.x+default-unsafe-string. For this case I would consider checking for the value of default_safe_string via ocamlc -config first.

[jeremiedimino]

Changing the default is likely to break a lot of stuff

[kit-ty-kate]

So will dune 1.0 :)

[jeremiedimino]

With dune, ocaml support will be a plugin (a builtin one, #461 ). So I guess for such changes, we'll simply bump the plugin version.

[c-cube]

People could still add unsafe-string to their list of flags, but I'd argue that a package that is not safe-string compliant in 2018 is somewhat broken anyway 😏

[jeremiedimino]

Yh, I agree that it should become the default, but we can't change it silently or it will break a lot of released packages in opam. It seems fine to change the default during the migration to Dune though