Martin, thanks for updating the patch. I've left some review comments on Rietveld.  After reviewing it, I think Garrett's original specification is correct: there is a need for four options to preserve current expected behavior although the default is slightly more complicated than stated.  The current behavior is: enable building readline with GNU readline if found on the search paths, possibly modified by CPPFLAGS and LDFLAGS, and, if not found and on OS X, use editline if found on the search paths.  Some build scripts depend on that behavior.