xapian_requirements should report Xapian bindings version

When Xapian is properly installed, the status report is blank. This patch changes it to report the Xapian bindings version.

At a future time, we may want to actually test the Xapian version to enforce a modern version, but I'm not sure what version is required. 1.0.6 is the latest, but 1.0.5 ships with Ubuntu's Hardy Heron and it works fine.