xdg-user-dirs: make sure libiconv is linked

Because the Darwin SDK automatically propagates libiconv, it is no
longer linked automatically (because that would link libiconv to
everything whether it wanted it or not).

xdg-user-dirs is failing to link not because it assumes libiconv comes
with libc but because of a bug in its build system. The `configure`
script detects how to link libiconv, but `Makefile.am` does not make use
of that to link libiconv when it is needed.

pkgs/by-name/xd/xdg-user-dirs/package.nix

@@ -1,11 +1,11 @@
 {
   lib,
   stdenv,
+  autoreconfHook,
   fetchurl,
   libxslt,
   docbook_xsl,
   gettext,
-  libiconv,
   makeWrapper,
 }:
 
@@ -18,9 +18,13 @@ stdenv.mkDerivation (finalAttrs: {
     hash = "sha256-7G8G10lc26N6cyA5+bXhV4vLKWV2/eDaQO2y9SIg3zw=";
   };
 
-  buildInputs = lib.optionals stdenv.hostPlatform.isDarwin [ libiconv ];
+  postPatch = ''
+    substituteInPlace Makefile.am \
+      --replace-fail 'libraries = $(LIBINTL)' 'libraries = $(LIBICONV) $(LIBINTL)'
+  '';
 
   nativeBuildInputs = [
+    autoreconfHook
     makeWrapper
     libxslt
     docbook_xsl