[ticket/11545] Remove DIRECTORY_SEPARATOR dependency from is_absolute

The given path is an absolute path in general, just not on our current system.

PHPBB3-11545
Merge remote-tracking branch 'nickvergessen/ticket/11545' into develop-olympus

* nickvergessen/ticket/11545:
  [ticket/11545] Remove DIRECTORY_SEPARATOR dependency from is_absolute
Merge branch 'develop-olympus' into develop

* develop-olympus:
  [ticket/11545] Remove DIRECTORY_SEPARATOR dependency from is_absolute