ldmud: fix MacOS iconv link time errors

The LDMud autotools build needs some help finding the right libiconv on
MacOS systems. Without nudging it the right direction with LDFLAGS the
system libiconv is used and produces missing symbol errors at link-time.

pkgs/by-name/ld/ldmud/package.nix

@@ -87,10 +87,14 @@ stdenv.mkDerivation (finalAttrs: {
     (lib.enableFeature pythonSupport "use-python")
   ];
 
-  preConfigure = lib.optionalString mysqlSupport ''
-    export CPPFLAGS="-I${lib.getDev libmysqlclient}/include/mysql"
-    export LDFLAGS="-L${libmysqlclient}/lib/mysql"
-  '';
+  preConfigure =
+    lib.optionalString mysqlSupport ''
+      export CPPFLAGS="-I${lib.getDev libmysqlclient}/include/mysql"
+      export LDFLAGS="-L${libmysqlclient}/lib/mysql"
+    ''
+    + lib.optionalString stdenv.hostPlatform.isDarwin ''
+      export LDFLAGS="$LDFLAGS -L${libiconv}/lib -liconv"
+    '';
 
   installTargets = [
     "install-driver"