libxml2: use libiconv on Darwin

This fixes e.g. python3Packages.beautifulsoup4, which has tests relying
on the wider encoding support enabled by libiconv.

Fixes #137678.

pkgs/development/libraries/libxml2/default.nix

@@ -1,5 +1,6 @@
 { stdenv, lib, fetchurl, fetchpatch
 , zlib, xz, libintl, python, gettext, ncurses, findXMLCatalogs
+, libiconv
 , pythonSupport ? enableShared && stdenv.buildPlatform == stdenv.hostPlatform
 , icuSupport ? false, icu ? null
 , enableShared ? stdenv.hostPlatform.libc != "msvcrt" && !stdenv.hostPlatform.isStatic
@@ -52,7 +53,9 @@ stdenv.mkDerivation rec {
     # RUNPATH for that, leading to undefined references for its users.
     ++ lib.optional stdenv.isFreeBSD xz;
 
-  propagatedBuildInputs = [ zlib findXMLCatalogs ] ++ lib.optional icuSupport icu;
+  propagatedBuildInputs = [ zlib findXMLCatalogs ]
+    ++ lib.optional stdenv.isDarwin libiconv
+    ++ lib.optional icuSupport icu;
 
   configureFlags = [
     "--exec_prefix=$dev"

pkgs/stdenv/darwin/default.nix

@@ -458,7 +458,7 @@ rec {
           libxml2 gettext sharutils gmp libarchive ncurses pkg-config libedit groff
           openssh sqlite sed serf openldap db cyrus-sasl expat apr-util subversion xz
           findfreetype libssh curl cmake autoconf automake libtool ed cpio coreutils
-          libssh2 nghttp2 libkrb5 ninja brotli;
+          libssh2 nghttp2 libkrb5 ninja brotli libiconv;
 
         "${finalLlvmPackages}" = super."${finalLlvmPackages}" // (
           let

pkgs/top-level/all-packages.nix

@@ -17939,6 +17939,7 @@ with pkgs;
 
   libxml2 = callPackage ../development/libraries/libxml2 {
     python = python3;
+    inherit (darwin) libiconv;
   };
 
   libxml2Python = let