We were looking at HADDOCK_DOCS instead of $$(HADDOCK_DOCS)

We were removing ".$(ProjectPatchLevel)" from anywhere in the file.
However, it included absolute paths, so if you untar a source
tarball into its default directory name, e.g.
"6.11.$(ProjectPatchLevel)", then the sed would break the paths.

We now also need to cast the values to (unsigned long), as on some
platforms sizeof returns (unsigned int).

Rather than using the autoconf built-in stuff and mangling it, we
now just ask the bootstrapping compiler what platform we are on.

When doing a port of GHC, you need to specify the platform you are
porting to.

The minimum version of GHC required is now 6.8.

Now that the clean rules don't require libraries/*/ghc.mk, we don't
have to build them on demand.  And having them built on demand
introduced a failure mode (where some libraries have ghc.mk and some
don't).

Overwriting means we lose the -m64 on OS X 64.

Without -shared, the default target is a binary and in that case
it makes sense for --make to not try and link a binary when there
is no Main module. But for a shared library the user already has
to specify -shared and there's no reason a shared lib should
contain any Main module or main function.

It tried to do this already, but the make variable name was wrong.

This is now the same as the old default. Currently we cannot embed
rpaths because they would point to the build tree. We should embed
rpaths relative to the $ORIGIN in future.