Support for the ARM 64 bit CPU architecture (aarch64) was introduced in autoconf 2.69. xz appears to use an earlier version of autoconf, preventing its being built. This can be fixed in of three ways (In order of preference): 1. Work with upstream to migrate the package to autoconf 2.69. 2. Rerun autoconf or autoreconf in %prep or %build prior to running configure. 3. Apply the patch at http://ausil.fedorapeople.org/aarch64/xz/xz-aarch64.patch which updates config.guess and config.sub to recognize aarch64.
This package uses %configure - so the config.{sub,guess} issue should be solved by the bug 951442.