Haddock version during build

[Manuel M T Chakravarty]

I just had a validate run halt with the message

   Running Haddock for ghc-prim-0.1...
   Setup: Haddock's internal GHC version must match the configured GHC  
version

after updating to Haddock 2.0.  I am far from convinced that it makes  
sense to enforce matching compiler versions here.  In particular, if  
this is enforced, Haddock would need to build as part of a standard  
GHC build, which seems like a waste of time.

Manuel