A bug crept in when s4u2proxy was added that basically crippled configuration via GssapiCredStore options if GssapiUseS4U2Proxy was not turned on. This forced people to set KRB5_KTNAME in the process environment to be able to use the keytab.
Documentaiton on how to use and install mod_auth_gssapi as missing, added a resonable start to the README file.