Tomi convinced me (with his review of my previous attempt) to go the
route of testing the default xapian backend once in configure, and
using that variable elsewhere.
This obsoletes the series(s)
id:1459855082-5715-2-git-send-email-david at tethera.net
id:1459938431-28670-2-git-send-email-david at tethera.net