Fix building on Mac OS X

The X11 headers are no longer in a "standard location", since
the X11 server is no longer provided directly by Apple.
Mac OS X: Ensure that SDL can be found when embedded in Wings

In Wings, the directory structure is the following:

  bin/beam.smp
  Frameworks/SDL.framwork

Adding this -rpath option ensures that the SDL framework can be
found in the same place relative to the BEAM emulator.