To cut a long story short, udunits code is a little old-ish and its Makefiles would need an overhaul. I have made a local package you can find here for now, and would like for someone to adopt this and make it a proper Debian package. The Makefiles needs a rewrite: only a static library is built, prefix indirection doesn't work completely, the manual pages go into the old locations of /usr/man and possibly more little things.
So the package is there, I'll put it into the next Quantian release, but I can't maintain it beyond that. This would make a nice project for a sciences grad student who actually uses NetCDF, and hence udunits.