This is a nginx client for JA-SIG CAS, a popular enterprise single sign-on solution.
nginx does not support dynamic loading of modules so you need to add this module to nginx's build time dependencies.
cd /path/to/nginx-source
./configure --add-module=/path/to/ngx_http_auth_cas_module
make install
This module is undergoing active development and its configuration options haven't quite stabilized yet.