Follow build system changes in integer-gmp

Also a couple of other fixes and sanity checks along the way.
Add deriveConstants to the dep files we build in phase 0

Dependency problem was discovered by int-e.

I've also added some comments about what's going on.
Remove ghc-pkg from the list of dep files we build during phase 0

I don't think it's necessary to build ghc-pkg that early.
Fix haddocking

We weren't seting the _DO_HADDOCK variables early enough.
Tweak the bindist comparison tool

It now just warns about files it doesn't recognise, rather than giving
an error. This means that random text files etc in the same directory
as the bindists don't make it fall over.