Provide the state of Cabal flags in an auto-generated module

(Imported from [Trac #836](http://hackage.haskell.org/trac/hackage/ticket/836), reported by lemming on 2011-04-24)

I would like to query the state of cabal flags at configure time without CPP hackery. I would prefer to import an autogenerated module, that contains Bool variables with the names of the Cabal flags, just like the Paths module contains the paths of installed data files.

For instance if my Cabal file contains

Flag buildExamples

  description: Build example executables

  default:     False

Flag buildTests

  description: Build test suite

  default:     True

Flag debug

  description: Compile with debug messages

  default:     False

and I install the package by

$ cabal install -fdebug

then the autogenerated module shall contain

module Flags_mypkg where

-- | Build example executables

buildExamples :: Bool

buildExamples = False

-- | Build test suite

buildTests :: Bool

buildTests = True

-- | Compile with debug messages

debug :: Bool

debug = True

Maybe this feature would also solve #778.