squid: add gnutls support

A new --without-gnutls option has been added to configure, so let's use
it in order to enable or disable gnutls support in squid.

Related:

  http://www.squid-cache.org/Versions/v3/3.5/squid-3.5.6-RELEASENOTES.html#ss4.1



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>