Update Cabal submodule to HEAD (1.21)

[Edward Z. Yang]

Since this patch causes GHC HEAD to not bootstrap out of the box
from GHC 7.6, I've reverted it for now.  We'll have to cross
this bridge sometime though.

Edward

Excerpts from Joachim Breitner's message of 2014-07-20 17:18:56 +0100:
> Hi,
> 
> Am Sonntag, den 20.07.2014, 17:08 +0100 schrieb Edward Z.Yang:
> > As long as we get Cabal to do a release before we cut a release,
> > it should be fine, so all we need is for Cabal to take the patch.
> 
> Nevermind, I noticed you updated the submodule to Cabal HEAD, not Cabal
> some-release.
> 
> 
> But anyways...
> 
> > Excerpts from Joachim Breitner's message of 2014-07-20 16:55:20 +0100:
> > > Am Sonntag, den 20.07.2014, 16:58 +0200 schrieb Joachim Breitner:
> > > So I got a working configuration. The following patch needs to be
> > > applied to Cabal:
> > > 
> > > [..]
> > > 
> > > and this adjustment made to GHCs build system:
> > > 
> > > [..]
> 
> doesn’t quite work. It finishes phase 0, but phase 1 says
> make[1]: *** No rule to make target 'libraries/process/dist-boot/build/System/Process.hi', needed by 'utils/hsc2hs/dist/build/Common.o'.  Schluss.
> and I’m stuck. Can someone help me here?
> 
> Thanks,
> Joachim
>