ASDF: alternative system configurations

How should I deal with alternative system configurations with ASDF? My
system can function both with or without CL-SDL, but the system is slightly
different when it is not present:

 window.lisp <-- needs SDL
 render.lisp <-- needs window.lisp if we have SDL

Currently I have solved this by having two alternative system definitions,
detecting the presence of SDL before calling ASDF, and then loading the
correct system. As far as solutions go it works, but is butt ugly.

Is there a way to handle this with a single system-definition?

Cheers,

  -- Nikodemus



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