macOS only supports extracting xz tarballs with `tar` in 10.9 and up.
GNU tar needs an `xz` executable on the `PATH` to extract xz tarballs.
(These are the most common variants of tar, so until further testing
is done, conservatively assume all variants of tar (other than the one
shipped with macOS) need an xz executable on the PATH in order to
decompress xz tarballs.)
Fixes#2155.
