Update xapian to new version 1.2.18

From http://xapian.org/docs/xapian-core-1.2.18/NEWS

API:
* Document: Fix get_docid() to return the docid for the sub-database (as it
  is explicitly documented to) for Document objects passed to functors like
  KeyMaker during the match.  (fixes#636, reported by Jeff Rand).
* Document: Don't store the termname in OmDocumentTerm - we were only using it
  in get_description() output and an exception message.  Speeds up indexing
  etext.txt using simpleindex by 0.4%, and should reduce memory usage a bit
  too.  (Change inspired by comments from Vishesh Handa on xapian-devel).
* Database: Iterating the values in a particular slot is now a bit more
  efficient for inmemory and remote backends (but still slow compared to
  flint, chert and brass).

testsuite:
* apitest: Expand crashrecovery1 to check that the expected base files exist
  and ones which shouldn't exist don't.
* queryparsertest: Fix testcase for empty wildcard followed by negation to
  enable FLAG_LOVEHATE so the negation is actually parsed.  Fortunately the
  fixed testcase passes.

matcher:
* OP_SYNONYM: avoid fetching the doclength if the weighting scheme doesn't
  need it and the calculated wdf for the synonym is <= doclength_lower_bound
  for the current subdatabase.  (fixes #360)

build system:
* Releases are now bootstrapped with libtool 2.4.2 instead of 2.4, and with
  config.guess and config.sub updated to the latest versions.

documentation:
* Add an example of initializing SimpleStopper using a file listing stopwords.
  (Patch from Assem Chelli)
* Improve the descriptions of the stem_strategy values in the API docs.
  (Reported by "oilap" on #xapian)
* docs/sorting.rst: Fix incorrect parameter types in Xapian::Weight
  subclass example.
* docs/glossary.rst: Add definition of "collection frequency".
* HACKING:
  + makeindex is now in Debian package texlive-binaries.
  + Replace a link to the outdated autotools "goat book" with a link to the
    "Portable Shell" chapter of the autoconf manual.
* include/xapian/base.h: Remove very out of date comments talking about atomic
  assignment and locking - since 0.5.0 we've adopted a "user locks" policy.
  (Reported by Jean-Francois Dockes)

examples:
* delve:
  + Add -A <prefix> option to list all terms with a particular prefix.
  + Send errors to stderr not stdout.
  + If -v is specified more than once, show even more info in some cases.
    (NEWS file claimed this was backported in 1.2.15, but it actually wasn't).
* quest:
  + Add --default-op option.
  + Add --weight option to allow the weighting scheme to be specified.

portability:
* Explicitly '#include <algorithm>' for std::max(), fixing build with VS2013.
  (Fixes#641, reported by "boomboo").
* Fix testcase blocksize1 not to try to delete an open database, which isn't
  possible under Windows.  (Fixes #643, reported by Chris Olds)
* docs/quickstart.rst: Split --cxxflags and --libs for portability (noted by
  "Hurricane Tong" on xapian-devel).
* Fix warnings with clang 5.0.

debug code:
* Add assertions that weighting scheme upper bounds aren't exceeded.