Use IDNA constants only if defined (ezyang#171)

Fixes ezyang#168.

Solution based on https://git.ispconfig.org/ispconfig/ispconfig3/commit/0e3cf6f51b4fd.

library/HTMLPurifier/AttrDef/URI/Host.php

@@ -97,7 +97,11 @@ public function validate($string, $config, $context)
 
         // PHP 5.3 and later support this functionality natively
         if (function_exists('idn_to_ascii')) {
-            $string = idn_to_ascii($string, IDNA_NONTRANSITIONAL_TO_ASCII, INTL_IDNA_VARIANT_UTS46);
+            if (defined('IDNA_NONTRANSITIONAL_TO_ASCII') && defined('INTL_IDNA_VARIANT_UTS46')) {
+                $string = idn_to_ascii($string, IDNA_NONTRANSITIONAL_TO_ASCII, INTL_IDNA_VARIANT_UTS46);
+            } else {
+                $string = idn_to_ascii($string);
+            }
 
         // If we have Net_IDNA2 support, we can support IRIs by
         // punycoding them. (This is the most portable thing to do,