xapian-omega: update to 1.4.25. Changes:

testsuite:

* omegatest.pl: Correct program name in error message.

build system:

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

* configure: Avoid compiler warning during GCC version check when compiler
  needs an option to enable C++11 support (same fix as applied to xapian-core
  in 1.4.23).

(schmonz)