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/Email.php b/system/libraries/Email.php
index 6a8bda7..967a001 100644
--- a/system/libraries/Email.php
+++ b/system/libraries/Email.php
@@ -1035,7 +1035,7 @@
 		if (function_exists('idn_to_ascii') && strpos($email, '@'))
 		{
 			list($account, $domain) = explode('@', $email, 2);
-			$domain = is_php('5.4')
+			$domain = defined('INTL_IDNA_VARIANT_UTS46')
 				? idn_to_ascii($domain, 0, INTL_IDNA_VARIANT_UTS46)
 				: idn_to_ascii($domain);
 			$email = $account.'@'.$domain;