cabal should pass extra lib/include dirs to configure scripts

(Imported from [Trac #754](http://hackage.haskell.org/trac/hackage/ticket/754), reported by simonmic on 2010-10-27)

cabal install SDL-gfx with macports has always failed for me, because that package's configure script doesn't know about the required extra lib dir /opt/local/lib. This made it work: LDFLAGS=-L/opt/local/lib cabal install SDL-gfx . cabal-install should probably use this or some other method to tell configure scripts about extra lib and include dirs.