Files that are only used by cabal setup are not recompiled when they change between runs of 'cabal build'

(Imported from [Trac #608](http://hackage.haskell.org/trac/hackage/ticket/608), reported by guest on 2009-11-15)

Files that are only used by cabal setup are not recompiled when they change between runs of 'cabal build'.

Reproduction steps for the included cabal package:

• Run 'cabal configure && cabal build && cabal test'
  
• The test output should be "I helped with lots of stuff!"
  
• Change the string in Distribution/Helper.hs to "I didn't help with "
  
• Rerun 'cabal configure && cabal build && cabal test' -- the output should change to "I didn't help with lots of stuff!", but nothing recompiles and the output doesn't change.
  

Although the attached cabal package is minimal, I've observed this behavior with darcs's Distribution/ShellHarness.