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- Package dph ist a meta package that contains a number of subpackages inside.
- This patch adds two targets to clean and remake all subpackages with a
  single command.

When cleaning or installing, we need to ignore what $stage is, and
just clean/install all the libraries.

It will use TH, so needs to be built with stage2.

This should stop us ending up without HsColour'ed sources on some
platforms.

We also now tell Cabal where to find HsColour, rather than it finding
it itself.

We need to forcibly use the in-tree Cabal, or we get version mismatch errors

Ifdefs for whether we had extensible exceptions or not were spreading
through GHC's source, and things would only have got worse for the next
2-3 years, so instead we now use an implementation of extensible
exceptions built on top of the old exception type.

They were causing some unnecessary work:
Running make in a built tree reregisters the GHC package in
bootstrapping.conf, and the build system thought that this updated
timestamp meant that the configure stamps were out of date. This is
particularly bad for the libraries with configure scripts, as those
take a while to run.

The bootstrapping.conf is built in an earlier phase ("make boot") so
one shouldn't rely on the dependencies anyway.

If DESTDIR is defined, then on *nix machines ghc-pkg is a script that
doesn't point to the right path. Therefore we use the ghc-pkg binary
directly.

One side-effect of this is that we need to build the install-utils with
stage2 rather than stage1 as we need the ghc package.

We can't do it when configuring, because we haven't built haddock yet