[racket] copy-shared-files and different platforms

At Mon, 25 Aug 2014 18:50:24 +0200, Jens Axel Søgaard wrote:
> > The "@loader_path" references mean that the shared libraries as
> > currently compiled work only when installed in the same scope as the
> > `racket/draw` libraries. Offhand, I don't have a good idea for solving
> > this problem. (FWIW, Windows installations won't have this problem.)

I've changed `raco setup` to replace "@loader_path" in installed Mach-O
files with an absolute path as needed.