patch applied (cabal): Only create includes and bin dirs if necessary

[Duncan Coutts]

Fri Aug 24 08:27:49 PDT 2007  Duncan Coutts <duncan at haskell.org>
  * Only create includes and bin dirs if necessary
  For includes, don't list the include dir in the package registration
  info unless we're actually going to install some include files.
  So we should create fewer pointless empty directories. See bug #97.

    M ./Distribution/Simple/Install.hs -8 +10
    M ./Distribution/Simple/Register.hs -1 +3