build: drop support for xapian versions less than 1.4

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.