Fix a bug where INTL_IDNA_VARIANT_UTS46 was used while undeclared

The constant was introduced in PHP 5.4, and is documented that way, but it actually depends on PHP being linked to ICU 4.6+, while the minimum requirement for compilation is 4.0 ...

When this happens on PHP 7.2, an E_DEPRECATED notice will be emitted; there's currently a discussion on php-internals to resolve that issue.
diff --git a/system/libraries/Form_validation.php b/system/libraries/Form_validation.php
index 3444c65..6a97ee5 100644
--- a/system/libraries/Form_validation.php
+++ b/system/libraries/Form_validation.php
@@ -1231,7 +1231,7 @@
 	{
 		if (function_exists('idn_to_ascii') && preg_match('#\A([^@]+)@(.+)\z#', $str, $matches))
 		{
-			$domain = is_php('5.4')
+			$domain = defined('INTL_IDNA_VARIANT_UTS46')
 				? idn_to_ascii($matches[2], 0, INTL_IDNA_VARIANT_UTS46)
 				: idn_to_ascii($matches[2]);
 			$str = $matches[1].'@'.$domain;