polkit: reintroduce check for pkcheck
    
    Commit 2499d1a0 was too eager and possibly enabled polkit
    on all platforms with D-Bus, regardless of whether they use polkit.
    
    Reintroduce the usage of pkcheck as a witness for --with-polkit=check,
    but do not require it for --with-polkit=yes.
    
    Signed-off-by: Ján Tomko <jtomko@redhat.com>
    Reported-by: Jiří Denemark <jdenemar@redhat.com>
    Reviewed-by: Andrea Bolognani <abologna@redhat.com>