use value of DIRECTORY_SEPARATOR to detect Windows

This commit unifies the detection of Windows builds across the Symfony
codebase.

Author: xabbuh

Core/Util/SecureRandom.php

@@ -43,7 +43,7 @@ public function __construct($seedFile = null, LoggerInterface $logger = null)
         $this->logger = $logger;
 
         // determine whether to use OpenSSL
-        if (defined('PHP_WINDOWS_VERSION_BUILD') && PHP_VERSION_ID < 50304) {
+        if ('\\' === DIRECTORY_SEPARATOR && PHP_VERSION_ID < 50304) {
             $this->useOpenSsl = false;
         } elseif (!function_exists('openssl_random_pseudo_bytes')) {
             if (null !== $this->logger) {