Support for the ARM 64 bit CPU architecture (aarch64) was introduced in autoconf 2.69. assimp 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/assimp/assimp-aarch64.patch which updates config.guess and config.sub to recognize aarch64.
assimp uses cmake, and should support aarch64 just fine. This bug was triggered by a bundled dependency's (cppunit) autotools scripts. the bundled cppunit isn't used in the package build process though, and i'm adding a step to %pre to remove the bundled cppunit sources just to be sure.