patch applied (ghc): Optionally use libffi to implement 'foreign import "wrapper"' (#793)

[Simon Marlow]

Thu Jan  3 09:02:36 PST 2008  Simon Marlow <simonmar at microsoft.com>
  * Optionally use libffi to implement 'foreign import "wrapper"' (#793)
  To enable this, set UseLibFFI=YES in mk/build.mk.  
  
  The main advantage here is that this reduces the porting effort for
  new platforms: libffi works on more architectures than our current
  adjustor code, and it is probably more heavily tested.  We could
  potentially replace our existing code, but since it is probably faster
  than libffi (just a guess, I'll measure later) and is already working,
  it doesn't seem worthwhile.
  
  Right now, you must have libffi installed on your system.  I used the
  one supplied by Debian/Ubuntu.

    M ./compiler/Makefile +5
    M ./compiler/deSugar/DsForeign.lhs -23 +67
    M ./rts/Adjustor.c -14 +100
    M ./rts/Makefile +5
    M ./rts/package.conf.in +3