compile problems on Mac OS X (with XCode 1.2 and BIND9)

What I'm doing in the attached patch is

1. check whether you're running Darwin
2. if so, test whether just including arpa/nameser.h gives you HEADER
3. if 2 fails, include arpa/nameser_compat.h and use -lresolv instead of

Thanks, I've integrated this and it'll be included in the next release.
My Darwin setup is evidently a bit unusual (since I didn't encounter the
problems that Ruoming & Brian did), so if others can please test this when
it goes out, I'd appreciate it.