Removing the custom mutex functions and (for sufficiently recent GnuTLS) the calls to gnutls_global_{,de}init() looks like a more and more attractive solution. I am not aware of anyone using OpenLDAP with GnuTLS on a platform for which GnuTLS lacks built-in mutex functions...

Just for the record: I recompiled libldap with the call to gnutls_global_set_mutex removed and cups doesn't segfault anymore.