Remove the need for xz as a prerequisite for builds on OS X.

On OS X the previous version of the build script packs a locally installed `xz` into the installation package and unpacks it during installation, if `xz` isn't available on the target machine. This can lead to problems if on the build machine `xz` was installed by a package manager like Homebrew (packing symlinks instead of the real binary).

The modification creates `xz` during the build and packs it into the installation package. `xz` is now longer needed for the build.