My 2 cents: I found I needed the developers versions of these for a complete successful ruby make (zlib-devel not zlib). installing rubygems required zlib-devel but I actually used ruby for a couple of weeks without realizing that readline had failed to make. My default linux (suse, 10.0) did not have the developers versions installed. In particular, check if your system has readline.h. If it doesn't, you don't have the developers readline installed.
Hope this helps someone!
