We were generating PrimopWrappers.hs twice in two different places,
one for the normal build and one for Haddock (I think).  Unfortunately
one of these was causing extra rebuilding, because the file dropped
into the dist-install/build/autogen directory, where it was picked up
by the automatic HS_SRCS detection in subsequent make runs.  This was
exposed by a recent change to HS_SRCS:

  Wed Oct 20 11:47:59 BST 2010  Ian Lynagh <igloo@earth.li>
    * Look for sources in Cabal's autogen directory too

This should fix the stage3 failures in the build bots.