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.

Update functions in Compat.Directory from originals in System.Directory

A tricky bug to do with the way the implication constraints are
solved in TcSimplify.  See Note [Reducing implication constraints].

mapAccumL and mapAccumR are in Data.List now.
mapAccumB is unused.

Use standard isPrefixOf/isSuffixOf instead.

$(GHC) is always the installed compiler, $(HC) changes depending on
the stage.  This has the nice effect that we build stage2/ghc-inplace
using the stage1 compiler, and get to take advantage of bugfixes in
stage1 relative to $(GHC) (in particular I want the non-blocking stdin
fix for stage2, otherwise piping the testsuite output through tee
fails).

The SIGCONT handler was used to put stdin back in O_NONBLOCK mode,
after the process is resumed, but we don't want to do that now: the
standard handles are kept in blocking mode.

(This required a bit of refactoring of CmmInfo.)