BUILD: Install krb5_child as suid if running under non-privileged user
    
    If sssd_be is running unprivileged, then krb5_child must be setuid to be
    able to access the keytab and become arbitrary user.
    
    Related:
    https://fedorahosted.org/sssd/ticket/2370
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>