Disable libIDN2 underscore workaround with libIDN or fixed libIDN2.

src/util.c

@@ -200,18 +200,20 @@ char *canonicalise(char *in, int *nomem)
   if (!(rc = check_name(in)))
     return NULL;
   
-#if defined(HAVE_IDN) || defined(HAVE_LIBIDN2)
-  /* libidn2 strips underscores, so don't do IDN processing
+#if defined(HAVE_LIBIDN2) && (!defined(IDN2_VERSION_NUMBER) || IDN2_VERSION_NUMBER < 0x02000003)
+  /* older libidn2 strips underscores, so don't do IDN processing
      if the name has an underscore (check_name() returned 2) */
   if (rc != 2)
+#endif
+#if defined(HAVE_IDN) || defined(HAVE_LIBIDN2)
     {
-#ifdef HAVE_LIBIDN2
+#  ifdef HAVE_LIBIDN2
       rc = idn2_to_ascii_lz(in, &ret, IDN2_NONTRANSITIONAL);
       if (rc == IDN2_DISALLOWED)
 	rc = idn2_to_ascii_lz(in, &ret, IDN2_TRANSITIONAL);
-#else
+#  else
       rc = idna_to_ascii_lz(in, &ret, 0);
-#endif
+#  endif
       if (rc != IDNA_SUCCESS)
 	{
 	  if (ret)