Fix deprecated gnutls_protocol_set_priority. (closes #3515)

Fix deprecated gnutls_protocol_set_priority.  (closes #3515)

This patch is based off of the patch at
http://dev.mutt.org/trac/attachment/ticket/3515/624058-gnutls-deprecated-set-priority.patch

The mutt source has changed since then, with more TLS options being
added.  This patch therefore uses a slightly different strategy for generating the
priority string.

The patch also adds autoconf support, as requested by brendan at
http://dev.mutt.org/trac/ticket/3515#comment:1