Build and use embedded xzdec for extracting app.tar.xz during installation.

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 `xzdec` during the build and packs it into the installation package. `xzdec` is now used *always* for unpacking `app.tar.xz`.