Fix DLYD problem with haskell libraries on Darwin

This commit fixes a problem that occurs with externally linked haskell
libraries on Darwin. It does this by adding the libraries to the
--extra-lib-dirs flag and the DYLD_LIBRARY_PATH environment variable.

pkgs/development/haskell-modules/generic-builder.nix

@@ -193,6 +193,9 @@ stdenv.mkDerivation ({
       fi
       if [ -d "$p/lib" ]; then
         configureFlags+=" --extra-lib-dirs=$p/lib"
+        if [[ ${ if stdenv.isDarwin then "yes" else "no"} = "yes" ]]; then
+          export DYLD_LIBRARY_PATH=$DYLD_LIBRARY_PATH:$p/lib;
+        fi
       fi
     done
     ${ghcCommand}-pkg --${packageDbFlag}="$packageConfDir" recache