Build: modernize Kerberos library detection
    
    Use package config instead of checking headers.
    Package config is faster because it does not invoke compiler
    and guarantees proper linking flags because these are provided
    by package maintainer instead of hardcoded into build system.
    
    libkrad does not have package config file so we keep the old way here.
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>