TLS1_3_VERSION needs to be ifdef'ed

[lindig]

Compilation of HEAD fails on openssl releases that don't define TLS1_3_VERSION. The code is in general careful by using HAVE_TLS13 et al to avoid this but this is not the case in

• https://github.com/savonet/ocaml-ssl/blob/master/src/ssl_stubs.c#L880-L914

A quick fix is in

• TLS1_3_VERSION must be ifdef'ed #69

but a more systematic approach would be beneficial.

[dinosaure]

I can confirm a compilation error on alpine-3.13 with OCaml 4.12.

[craigfe]

This issue is still failing some of our CI running on alpine-3.13.

Is it possible to cherry-pick 5cbeba7 from #63 and merge that as a separate change?

[psafont]

Fixed by #73