patch applied (ghc): Handle unbuildable packages differently

[Ian Lynagh]

Mon Apr 30 04:15:40 PDT 2007  Ian Lynagh <igloo at earth.li>
  * Handle unbuildable packages differently
  Now we require that configure fails, and use our own signalling rather
  than Cabal's buildable field.

    M ./libraries/Makefile -10 +10
    M ./libraries/ifBuildable.hs -26 +16