Could you please update c-ares in EL-5? Before 1.5.3, it does not provide a .pc/pkg-config file, so ZNC (among other programs) will not detect it.
c-ares-1.6.0-1.el5 has been submitted as an update for Fedora EPEL 5. http://admin.fedoraproject.org/updates/c-ares-1.6.0-1.el5
c-ares-1.6.0-1.el5 has been pushed to the Fedora EPEL 5 testing repository. If problems still persist, please make note of it in this bug report. If you want to test the update, you can install it with su -c 'yum --enablerepo=updates-testing update c-ares'. You can provide feedback for this update here: http://admin.fedoraproject.org/updates/EL-5/FEDORA-EPEL-2009-0118
c-ares-1.6.0-1.el5 has been pushed to the Fedora EPEL 5 stable repository. If problems still persist, please make note of it in this bug report.