problems building ghc 6.4.1_pre using 6.4 with multiplecabalversions installed

[Andres Loeh]

> Just glancing over the patch, I can't immediately see how it works.  GHC
> 6.4 gives priority to package modules over modules on the local search
> path, so Distribution.* will be taken from the installed Cabal package.
> However, when linking GHC you are ommitting -package Cabal, so I'd
> expect a link error.
> 
> Hmm, I guess I should try this and figure out what's happening.

The idea is that no installed Cabal version will be used. I think the
ghc distribution should just always build its own Cabal during stage1.
That's what the patch tries to achieve, and afaics, it works. However,
Duncan didn't give you the full story with the patch. We also do the
following:

echo "GHC+=-ignore-package Cabal" >> mk/build.mk
echo "HC+=-ignore-package Cabal" >> mk/build.mk

This was the only way I could find to make sure that a preinstalled
Cabal is ignored whenever the preinstalled ghc is called, but not once
the in-place ghc is available.

Cheers,
  Andres