Support for the ARM 64 bit CPU architecture (aarch64) was introduced in autoconf 2.69. libgcal 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/libgcal/libgcal-aarch64.patch which updates config.guess and config.sub to recognize aarch64.
Thank you for your report. The upstream is not really much active...I think it is a good idea to add a patch to the rpm and ask to the upstream to include it. I'll do it as soon as possible. Thank you!