Support for the ARM 64 bit CPU architecture (aarch64) was introduced in autoconf 2.69. xfce4-taskmanager 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/xfce4-taskmanager/xfce4-taskmanager-aarch64.patch which updates config.guess and config.sub to recognize aarch64.
xfce4-taskmanager-1.0.0-7.fc19 has been submitted as an update for Fedora 19. https://admin.fedoraproject.org/updates/xfce4-taskmanager-1.0.0-7.fc19
xfce4-taskmanager-1.0.0-7.fc19 has been pushed to the Fedora 19 stable repository. If problems still persist, please make note of it in this bug report.