That said, it *might* be OK to change the default behavior to just remove the "and on OS X" condition:  enable building readline with GNU readline if found on the search paths, possibly modified by CPPFLAGS and LDFLAGS, and, if not found, use editline if found on the search paths.  That's probably the desired behavior on FreeBSD systems where, AFAIK, GNU readline isn't shipped by default but BSD editline is.