Switch to the maintained Apple xar lineage (build 503, versioned
1.8.0.0.503): the mackyle 1.6.1 fork this package tracked has been
dead since 2012, and Debian, Fedora and Gentoo all moved to Apple's
xar (apple-oss-distributions/xar). This resolves the long-standing
NULL-pointer dereferences in xar_get_path() and xar_unserialize()
when parsing malformed archives: