> I have both on my box, and it looks as if the Makefile in ext/ 
> readline is using readline:
>
> LIBS = $(LIBRUBYARG_SHARED) -lreadline -lncurses  -lpthread -ldl - 
> lobjc

On Mac OS X, readline is a symbolic link to libedit:
% ls -al /usr/lib/libreadline.dylib
lrwxr-xr-x  1 root  wheel  15 Nov 10  2007 /usr/lib/libreadline.dylib - 
 > libedit.2.dylib