Add universal ppc/i386 Mac dylibs

These should work on anything from G3 up.
Simplify library loading code
Fix install names in libtcod.dylib

The previous path, just the 'libSDL.dylib' basename, meant it
wouldn't be found at load time. This instead uses:
@loader_path/libSDL.dylib

@loader_path is relative to the library requesting the load, ensuring that
this will be found regardless of path.

@loader_path was introduced in OS X 10.4, so it should be compatible
across versions. (I avoided @rpath for that reason.)