download/compile xz for building/decompressing tarball in OS X installer

On OS X, we previously packed a locally installed xz into the installation package
and unpacked it during installation, if xz is not available on the target machine.
This can lead to problems if xz was installed by a package manager like Homebrew,
as symlinks would get packed instead of the real binary.

We now download and compile xz statically on the fly, which removes the need for
xz on both the developer and the deployment machine.