Moved creation of symlink to postFixup phase.

This, together with a check if the symlink is necessary,
should prevent the occurrence of dangling symlinks.

svn path=/nixpkgs/trunk/; revision=24232

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

@@ -102,12 +102,15 @@
                 GHC_PACKAGE_PATH=$installedPkgConf ghc-pkg --global register $pkgConf --force
               fi
 
-              ensureDir $out/nix-support
-              ln -s $out/nix-support/propagated-build-native-inputs $out/nix-support/propagated-user-env-packages
-
               eval "$postInstall"
             '';
 
+            postFixup = ''
+              if test -f $out/nix-support/propagated-build-native-inputs; then
+                ln -s $out/nix-support/propagated-build-native-inputs $out/nix-support/propagated-user-env-packages
+              fi
+            '';
+
             # We inherit stdenv and ghc so that they can be used
             # in Cabal derivations.
             inherit stdenv ghc;