use value of DIRECTORY_SEPARATOR to detect Windows

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

[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