Linking, tls and windows

[dinosaure]

It seems we have a linking bug when we want to have the SSL support on cohttp. I really don't know what happens but, on ocaml-git, if I want to use tls, I did not have any problem on Linux but (cf. mirleft/ocaml-nocrypto#135) I have a problem on Windows.

So, a solution could be "(ssl | tls)" as a dependency of git-unix but neither Travis nor Appveyor is happy ... Then, with a little discussion with @samoht, the problem seems to be in conduit. So it's why I created this issue.

[samoht]

The cause is mirleft/ocaml-nocrypto#135

conduit.unix should stop linking to nocrypto.unix unconditionally as this will never work (at leats, until someone ports nocrypto.unix to windows)

[samoht]

We need to define tls as an optional dependency only on non-windows, but I am not sure it is possible with opam 1.2 syntax.