It appears that libIDL is packaged with old config.guess file and when you try to build it on Mac OS X 10.5 (Leopard) you will end up with this error:
configure: error: can not guess host type; you must specify one
An easy fix is to replace the config.guess and config.sub files before running ./configure:
cp /usr/share/automake-1.10/config.* ./
