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This is breaking :list for someone.  If there's a problem here, we
should fix it another way.

That call was and is commented out, but it serves as documentation
for when the CPS pass will be turned on.

On OS X, we have to #include <GMP/gmp.h> if we are using GMP.framework. Before
the recent GMP changes, gcc (incorrectly) used the gmp.h supplied by ghc but
that is gone now. 

gmp is now in a top-level directory and we only have the tarball in the
darcs repo. It gets untarred if it is needed.