require xapian >= 1.2.6

require xapian >= 1.2.6

It seems that no-one tried to compile without Xapian compact support
since March of 2015, since that's when I introduced a syntax error in
that branch of the ifdef.

Given the choice of maintaining this underused branch of code, or
bumping the Xapian dependency to a version from 2011, it seems
reasonable to do the latter.