Fix install names in libtcod.dylib
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.)

mispy/libtcod,remcopeereboom/libtcod