Make CEGUILua v0.5.0 Lua 5.1 compliant

[rgrieder]

There is a certain possibility (quite high actually) that one user might have CEGUI version 0.5.0 (currently the one found in various packages for Linux distros) and Lua version 5.1. Unfortunately CEGUILua v0.5.0 was not built to use Lua 5.1, but it could be easily modified by looking at the changes in CEGUILua v0.6.1.
The most important thing to keep in mind when dealing with CEGUILua is that we need to have the same lua library (not only the same version, but also the same loaded lib file) for both CEGUILua and our own stuff. Otherwise we might get serious unexplainable runtime errors.

[rgrieder]

Resolved the problem by adding support for versions 0.5.0, 0.6.0, 0.6.1 and 0.6.2. There is also a possibility to supply your own CEGUILua Library by activating the CEGUILUA_USE_EXTERNAL_LIBRARY CMake switch.