minor #13147 [2.3] for consistency, use value of DIRECTORY_SEPARATOR to detect Windows (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[2.3] for consistency, use value of DIRECTORY_SEPARATOR to detect Windows

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

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

Commits
-------

20a427d use value of DIRECTORY_SEPARATOR to detect Windows

Author: fabpot

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) {