Die hard on no xtables.h header file

[netoptimizer]

When calling:
perl Makefile.PL

It will detect if xtables.h is missing on the system, and print a WARNING message and let you continue.

Perhaps this should be an exit/die as automated CPAN testers will not see this message, and continue to hit the compile error.
Don't know how the CPAN auto test reporters react to this situation...