Support for the ARM 64 bit CPU architecture (aarch64) was introduced in autoconf 2.69. pure 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/pure/pure-aarch64.patch which updates config.guess and config.sub to recognize aarch64.
Current version of Pure uses a newer version of autoconf that fixes the issue, however, due to incomplete LLVM support for ARM archiectures building for all ARM variants is currently excluded. Since the specific issue here is no longer relevant I'm marking this as fixed.