patch applied (cabal): Make scratchdir into a proper configure flag

[Duncan Coutts]

Fri Nov 16 05:16:04 PST 2007  Duncan Coutts <duncan at haskell.org>
  * Make scratchdir into a proper configure flag
  rather than using the flag extension mechanism.
  It was the old flag using it and I'm about to replace that whole system.

    M ./Distribution/Simple.hs -8 +3
    M ./Distribution/Simple/Configure.hs -1 +2
    M ./Distribution/Simple/Setup.hs +6