patch applied (packages/Cabal): On windows, pick up ar.exe from the ghc install before looking on the $PATH

[Ian Lynagh]

Sat Mar 20 07:36:43 PDT 2010  Duncan Coutts <duncan at haskell.org>
  * On windows, pick up ar.exe from the ghc install before looking on the $PATH
  Some ar.exe versions floating around seem to have weird non-posix behaviour.

    M ./Distribution/Simple/GHC.hs -5 +13