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

[lemming]

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.

[elga]

*  simyo