Hook external/public-domain/tz into the build, and unhook share/zoneinfo.

The timezone files are still installed in ${DESTDIR}/usr/share/zoneinfo/**,
and they are identical before and after this change.

This is almost the last step in moving NetBSD's tzdata sources from
share/zoneinfo to external/public/tz/dist, to ease future maintenance.
All that remains is to delete src/share/zoneinfo, but that will be
delayed for a while.

branches: 1.2.8; 1.2.18;
Hook up SQLite into the build

Hook up xz into the build system.

Rebase.

Rebase to HEAD as of a few days ago.