Support for the ARM 64 bit CPU architecture (aarch64) was introduced in autoconf 2.69. krb5-appl 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/krb5-appl/krb5-appl-aarch64.patch which updates config.guess and config.sub to recognize aarch64.
The autoconf 2.69 package in F19 doesn't include a config.guess or config.sub for it to pick up, and we already use option 2, so I don't think option 1's going to change anything. Based on traffic on the devel list, I'm guessing we either need to rope in a newer automake and grab the files from there, or to start packaging the configs directly. Upstream uses a pretty recent autoconf (2.68 as of krb5 1.11.1), so I guess carrying option 3 until a subsequent upstream release picks up a sufficiently-recent config.guess/config.sub combination is the way to go, then.
Please ignore that second paragraph; it was meant for bug #925640.