patch applied (cabal): Add impl(...) conditional to configurations.

[Duncan Coutts]

Wed Aug  1 18:05:27 PDT 2007  Thomas Schilling <nominolo at gmail.com>
  * Add impl(...) conditional to configurations.
  You can now use expressions like impl( ghc >= 6.6.1 ) in conditionals
  in .cabal files.

    M ./Distribution/Configuration.hs -8 +20
    M ./Distribution/PackageDescription.hs -6 +8
    M ./Distribution/Simple/Configure.hs -1 +2