ffd5b7d7ab
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.