[ticket/12762] Make ext.php optional for extensions

PHPBB3-12762
Merge pull request #2642 from Nicofuma/ticket/12762

[ticket/12762] Make ext.php optional for extensions

* Nicofuma/ticket/12762:
  [ticket/12762] Make ext.php optional for extensions
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12762] Make ext.php optional for extensions