% ls -la /usr/lib/libreadline.dylib
lrwxr-xr-x 1 root wheel 15 15 Sep 2009 /usr/lib/libreadline.dylib@ -> libedit.2.dylib
If you just configure your PostgreSQL build without any special options, the PostgreSQL binaries will be linked against libedit.
There were several discussions about this in the past and this was even reported to Apple. However, since now (i'm on OSX 10.6.6 for example), nothing happened and the shipped libedit is still unusable with psql.
If you really need to build your own PostgreSQL binaries on OSX (for example you want do a review for a patch in the current CommitFest ;)), i always recommend to use MacPorts and their packaging. I use it for nearly two years and it works quite well (well, at least all libraries required by my installation are working). There's a fairly good instruction on how to setup MacPorts on your MacBook. If you have done your package installation, simply point the configure script to the correct location of the libraries and header files:
./configure --with-libraries=/opt/local/lib --with-includes=/opt/local/include ...
/opt/local is the default location for libraries installed from MacPorts. You also might want to specify additional options. At least, --with-includes and --with-libraries are necessary to point your PostgreSQL builds to the correct libraries.