iptables plugin: Support the new libiptc API.

iptables plugin: Support the new libiptc API.

When libiptc has been officially made available as a shared library, the API
and ABI have been changed slightly. By checking for the existance of a type
that has been removed in that course, configure now checks which version is
available. This is quite error prone (the type might be re-introduced any
time), so this should be improved some time - currently, I do not have an idea
how to do so, though :-/