use value of DIRECTORY_SEPARATOR to detect Windows

This commit unifies the detection of Windows builds across the Symfony
codebase.
Merge branch '2.3' into 2.5

* 2.3:
  [Form] Remove a redundant test.
  use value of DIRECTORY_SEPARATOR to detect Windows

Conflicts:
	src/Symfony/Component/Console/Application.php
	src/Symfony/Component/Filesystem/Tests/FilesystemTest.php
	src/Symfony/Component/Form/Tests/Extension/Csrf/Type/FormTypeCsrfExtensionTest.php
	src/Symfony/Component/Process/Process.php
Merge branch '2.5' into 2.6

* 2.5:
  [Form] Remove a redundant test.
  use value of DIRECTORY_SEPARATOR to detect Windows

Conflicts:
	src/Symfony/Component/Process/Process.php
Merge branch '2.6' into 2.7

* 2.6:
  [Form] Remove a redundant test.
  use value of DIRECTORY_SEPARATOR to detect Windows