polkit: reintroduce check for pkcheck

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>