it depends on where you installed readline when you built it, and whether the PG configure was able to find it there - the install location is set with --prefix=/path/to/install when you configure readline, and you can tell the PG configure where to find it with --with-includes=DIRS and --with-libraries=DIRS
Personally, I use fink for infrastructure type stuff like readline, and configure all my local software with "--with-includes=/sw/include --with-libraries=/sw/lib" and stuff tends to work without manual massaging...
From the other errors you're seeing, it looks like you may be using a version of PG that must be patched to work with OSX >= 10.2. What version of PG are you trying to build? If it's 7.2, you'll need to apply this patch to your source tree (it's against 7.2.1, but I'd expect it'll apply cleanly against whatever is at the head of 7.2 these days)
cheers
