Some libs are really missing. They have not be compiled for the wl500g ATM.
In the case of thttpd, you probably need to install the package zlib. Normaly, a pckage check and download it's dependency. In this case, it is probably not completly setup, as it as been added separatly from the normal unslug repository. Try an "ipkg.sh list" to try and search the missing libs. I can garantee you the libs are here. You can try also using ipkg instead of ipkg.sh after installing it's package. (ipkg.sh isntall ipkg)
Don't forget also to do in ipkg.sh update (or ipkg update, depending on what you are using) to get the latest version