1. The Overlay headers are installed in include/Ogre/OgreOverlay. The OGRE-Overlay.pc.in template file sets the include path to include/Ogre/Overlay.
2. The Overlay pkg-config file is not installed.
3. OgreHeaderPrefix.h / OgreHeaderSuffix.h are not installed, even though they are Windows only they are still included from other headers.
Original Mantis Ticket: http://www.ogre3d.org/mantis/view.php?id=578