Log Message:
xapian-omega: update to 1.4.27. Changes:

omega:

* Calculate date spans in days rather than converting to time_t, which
  side-steps issues due to 32-bit time_t and some implementations not
  handling negative time_t values.

portability:

* Fix build with UCRT64 variant of mingw-w64 by stopping defining
  __MSVCRT_VERSION__ by default. We fixed this for xapian-core in 1.4.24
  but missed that omega defined it too.

* Remove unnecessary 'using namespace std' to fix build on (at least)
  FreeBSD where's nothing in the std namespace to import at this point
  so we get a compiler error.