haskell-packages.nix: improved handling of 'Cabal'

Most packages use the Cabal library only as a build tool, and as such it is a
dependency of the cabal.nix builder -- not of the package that's being built
with that builder.

svn path=/nixpkgs/trunk/; revision=32326

pkgs/development/libraries/haskell/cabal/cabal.nix

@@ -1,6 +1,6 @@
 # generic builder for Cabal packages
 
-{stdenv, fetchurl, lib, pkgconfig, ghc, enableLibraryProfiling ? false} :
+{stdenv, fetchurl, lib, pkgconfig, ghc, Cabal, enableLibraryProfiling ? false} :
 {
   mkDerivation =
     args : # arguments for the individual package, can modify the defaults
@@ -51,7 +51,7 @@
             # default buildInputs are just ghc, if more buildInputs are required
             # buildInputs can be extended by the client by using extraBuildInputs,
             # but often propagatedBuildInputs is preferable anyway
-            buildInputs = [ghc] ++ self.extraBuildInputs;
+            buildInputs = [ghc Cabal] ++ self.extraBuildInputs;
             extraBuildInputs = self.buildTools ++
                                (if self.pkgconfigDepends == [] then [] else [pkgconfig]) ++
                                (if self.isLibrary then [] else self.buildDepends ++ self.extraLibraries ++ self.pkgconfigDepends);