* As predicted, returning {} if a package isn't supported on a

  platform won't work: nix-env -qa barfs with "error: cannot coerce an
  attribute set (except a derivation) to a string".  So throw an error
  instead.  Anyway, condPackage/platformPackage doesn't belong in
  all-packages.nix (it should be specified in the package itself).

svn path=/nixpkgs/trunk/; revision=16870