[2.6] for consistency, use 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.

Changes specific to Symfony 2.6 and not part of neither #13147 nor #13148.
shouldn't this become ``'\\' === DIRECTORY_SEPARATOR``?