I tried adding the following to the Makefile:

.if ${OPSYS} == "Darwin"
CONFIGURE_ARGS+=        --with-installed-readline
.endif

..however, this results in:

checking version of installed readline library... ./configure: line 
4521: test: too many arguments
EditLine wrapper
configure: WARNING: installed readline library is too old to be linked 
with bash
configure: WARNING: using private bash version

- Frank