[wplug] Very strange problem

[Jack]

you could always try specifying an abslute path to ip.h

sounds to me like maybe there's another netinet/ip.h path on your sys that's being used?  not likely, but it sure sounds like it--and the one it's using doesn't have those things def'd.  could be why renaming it to something unique--while still in the same relative path spec--results in proper inclusion.  might be something to try.

#include </usr/include/netinet/ip.h