There was a GSOC project for porting mDNSResponder for Haiku a long time ago (2009 if I remember right). If you can find the code / patches for that it might help you get started, but the code is some 15 years old and probably bitrotten quite a bit. But still, it might be helpful for finding out how the GSOC student got it to compile back then.
edit: The results of the GSOC 2009 are here: Google Code Archive - Long-term storage for Google Code Project Hosting.