amd64/libtcod.dylib: fix SDL install name

SDL was incorrectly referred to via the install name libSDL.dylib,
which caused it to be looked up in the default path, ignoring the
bundled libSDL in the same directory. The name has been changed to
@loader_path/libSDL.dylib, which will pick up the bundled copy.
(This matches the install name for the PowerPC and i386 dylibs.)
Merge pull request #11 from mistydemeo/install_names

amd64/libtcod.dylib: fix SDL install name