Support for the ARM 64 bit CPU architecture (aarch64) was introduced in autoconf 2.69. libpng10 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/libpng10/libpng10-aarch64.patch which updates config.guess and config.sub to recognize aarch64.
libpng and libpng10 have the same upstream and build from the same general codebase so I'll leave the upstreaming to the libpng maintainer. There would only be another libpng10 release in the event of a security issue in all probability. In the meantime, I have applied Dennis' patch and done builds for F-19 and Rawhide.