Support for the ARM 64 bit CPU architecture (aarch64) was introduced in
autoconf 2.69. gtkglext 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
3. Apply the patch at http://ausil.fedorapeople.org/aarch64/gtkglext/gtkglext-aarch64.patch
which updates config.guess and config.sub to recognize aarch64.