Do additional ghc / ghc-pkg consistency check

(Imported from [Trac #699](http://hackage.haskell.org/trac/hackage/ticket/699), reported by @dcoutts on 2010-06-05)

See [http://hackage.haskell.org/trac/ghc/ticket/4103](http://hackage.haskell.org/trac/ghc/ticket/4103)

In future, ghc --info will report the global package database location. This can be used to check that ghc and ghc-pkg agree on the same one, as a consistency check to make sure the two programs are from the same ghc installation.

Note that getting the location of the global package db from ghc-pkg is a bit hacky at the moment. One has to use ghc-pkg list --global and parse the result. Perhaps we need another ghc feature request for that.