Is the bug just that there wasn't a failure?  Building programs from
source on a system without bison/yacc seems like an odd edge case.  I'd
be in favor of just making configure fail if not found.  yacc has been
part of the base unix system for a long long time at this point, and
surely it isn't hard for people to install, even on distributions that
split out the various tools (which is fine - but I suppose we need to
list it explicitly in the dependencies file).