Update to rra-c-util 3.6

Update to rra-c-util 3.6

* Check for krb5-config in /usr/kerberos/bin as well as user's PATH.
* Add replacement for krb5_appdefault_* functions for AIX Kerberos.
* Fix broken GCC attribute markers.
* Fix Kerberos library probing without transitive shared libraries.
* Suppress warnings when probing for AIX-only Kerberos headers.
* Support Heimdal GSS-API on OpenBSD without a separate libroken.
* Update GCC warning flags for GCC 4.6.1.