Xapian 1.4 is over 3 years old now (1.4.0 released 2016-06-24),
and 1.2 has been deprecated in Notmuch version 0.27 (2018-06-13).
Xapian 1.4 supports compaction, field processors and retry locking;
conditionals checking compaction and field processors were removed
but user may want to disable retry locking at configure time so it
is kept.
Many of the external links found in the notmuch source can be resolved
using https instead of http. This changeset addresses as many as i
could find, without touching the e-mail corpus or expected outputs
found in tests.
