Provide Mac PowerPC dylibs, improve loader code

This PR includes a few changes:

* Adds a set of 32-bit PowerPC libraries. These were built/tested on 10.5 and work well. They're provided as a PowerPC slice to the 32-bit Intel libraries, but there's also a `powerpc` directory with symlinks to the universal libraries.
* Cleans up the lib loading code a little bit to reduce redundancy.
* Fixes the install_names to the 32-bit OS X libtcod library, which fixes an issue where SDL couldn't be found by the dynamic loader. I can't test the 64-bit library at the moment.