Support for the ARM 64 bit CPU architecture (aarch64) was introduced in autoconf 2.69. krb5 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/krb5-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.
Should be sorted in krb5-1.11.1-5.fc20 and later. Please reopen if you find that this is not the case.