patch applied (cabal-install): Don't add installed constraints system packages that are not installed

[Duncan Coutts]

Wed Jan 14 06:35:40 PST 2009  Duncan Coutts <duncan at haskell.org>
  * Don't add installed constraints system packages that are not installed
  In particular fixes a problem (ticket #439) where we required the
  installed version of ghc-prim with compilers that do not have that
  package such as ghc-6.8 and older, hugs, nhc, lhc etc.

    M ./Distribution/Client/Dependency.hs -4 +4

View patch online:
http://darcs.haskell.org/cabal-install/_darcs/patches/20090114143540-adfee-c114f609d4730fb69520338fd095ee036fc0790f.gz