Log Message:
xapian-omega: update to 1.4.26. Changes:

indexers:

* omindex:

  + Make robust to the indexer process being run with stdin or stdout closed.

omega:

* Support "bm25+" and "pl2+" in "$set{weighting,...}".

* Deprecate "lm" in "$set{weighting,...}".  This was meant \ 
to implement the
  "Language Model" Weighting scheme, but we've discovered the \ 
implementation
  was incorrect and fixing it requires ABI-incompatible changes in xapian-core.
  For 1.4.x we need to leave it in place so as not to break existing code, but
  we recommended avoiding using it.  It will be removed in the next release
  series and replaced with new separate classes implementing Language Model
  weighting with each smoothing.

* Add "prob" as new preferred name for probabilistic query expansion in
  "$set{expansion,...}", with the previous "trad" still \ 
being accepted for now.

build system:

* Report result of probe to determine compiler support for -Werror or
  equivalent.

* If pkg-config is available, use it to probe for libmagic.

* configure: Probe for closefrom().  Patch from Qiu Yingbo in
  https://github.com/xapian/xapian/pull/323

portability:

* configure: Fix clang detection which wasn't working when configure determined
  a -std=X option was needed to get C++11 support.  The obvious symptom was
  that --enable-werror wouldn't add -Werror.

* configure: NetBSD automatically pulls in library dependencies, so set
  link_all_deplibs_CXX=no there.

* Define __WIN32__/__WIN64__ like we do for xapian-core.  Spotted by Baran Demir.

* Avoid using sprintf() if snprintf() is available, even in cases where the
  output size is bounded to avoid deprecation warnings on macOS.  For 1.4.x
  we still fall back to sprintf() to avoid a point release breaking support
  for any platform still lacking snprintf().

* Use `override` for subclassing functors.  This is good practice as it gives a
  clear compile error if we have to change the signature of an virtual method
  on such a functor.  See #830.

* Fix building with MSVC - it seems to support AR=lib we need to use AM_PROG_AR
  which probes for AR's command line interface.