Add OGRE-Overlay plugin

Since OGRE 1.9, the overlay plugin is separated from the main OGRE
library.

Add OGRE-Overlay to configure.ac and src/Makefile.am and regenerate
autotools files.
Fix compilation issues with OGRE 1.9.0

* Add missing semicolons after OGRE_LOCK_MUTEX
* Include OgreOverlayManager.h
* Initialize the newly-separated Overlay System with doc from
  http://www.ogre3d.org/tikiwiki/tiki-index.php?page=Manual+Resource+Loading#Overlay_2
Arch Linux: improve PKGBUILD
Arch Linux: update PKGBUILD