bd9cab6fb4
libarchive(3) doesn't support the new liblzma API yet, but this change allows us to enable multi-threaded xz compression. ``make release'' should now finish in half the time on a machine with several cores and fast disks (our typical build server). This behaviour only applies when building a release and it doesn't affect buildworld/installworld. To disable threaded xz compression, set XZ_THREADS=1. Reviewed by: gjb Tested by: gjb