gettext: fix build on darwin

Hash on Linuxes is unchanged. For details about the error see discussion at:
https://github.com/NixOS/nixpkgs/commit/0ffb98828e87612

pkgs/development/libraries/gettext/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, libiconv, xz }:
+{ stdenv, fetchurl, libiconv, libintlOrEmpty, xz }:
 
 stdenv.mkDerivation (rec {
   name = "gettext-0.18.2";
@@ -30,7 +30,8 @@ stdenv.mkDerivation (rec {
     fi
   '';
 
-  buildInputs = [ xz ] ++ stdenv.lib.optional (!stdenv.isLinux) libiconv;
+  buildInputs = [ xz ]
+    ++ libintlOrEmpty ++ stdenv.lib.optional (!stdenv.isLinux) libiconv;
 
   enableParallelBuilding = true;