bug#18526: Failure to download from github due to TLS fatal alert

[Ludovic Courtès]

Commit 077bd18 fixes it, using the ‘set-session-server-name!’ procedure
introduced in GnuTLS commit e9fc746:

  
https://gitorious.org/gnutls/gnutls/commit/e9fc74641dae2747b98bc9f79afc041805819339

It should be in the next GnuTLS release.

Ludo’.