Why would it be help2man's business to deal with cross-compiles? Agreed.
IMO it is configure's business. What do you think about this patch? Seems ok to me, but I wonder whether any test is really necessary. As far as I know coreutils, texinfo, and other packages have used help2man for years, and I have never seen a bug report about the problem of help2man failing on cross compiles ... I don't like complicating Hello (and every other package) with a theoretical "problem". Hmm.