Rationale: some software (e.g. GNU Coreutils, GnuTLS 3.x), have started
distributing _only_ xz-compressed tarballs. There is no system XZ
utility provided by OS X, but it is necessary so that we can continue to
provide formulae for this software.
If XZUtils isn't installed, we abort and prompt the user to `brew
install xz`.
The `xz` command itself doesn't do any untarring, so we write the
decompressed archive to stdout and pipe it to tar.
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