Support for the ARM 64 bit CPU architecture (aarch64) was introduced in autoconf 2.69. xpilot-ng 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/xpilot-ng/xpilot-ng-aarch64.patch which updates config.guess and config.sub to recognize aarch64.
%configure from redhat-rpm-config >= 9.1.0-42 updates config.guess and config.sub for new architecture support, and this packages spec-file uses %configure, closing.