[ocaml-platform] "opam config env" and emacs

[Jeremy Yallop]

An alternative approach is to change opam so that it maintins a
symlink to the current compiler, avoiding the need for an
emacs-specific solution.  This approach works well for Debian's
'alternatives' system, for example, and also has the advantage that
there's no need to run a second command to update the environment
after switching compilers.

I see that Raphael Proust already has a pull request along these lines:

https://github.com/OCamlPro/opam/pull/477