Linux support

Hi,
I've restored linux support with CMake.
I made my best not to break msvc build but apparently our environments are slightly different (I used VC++ 2010 express). I had to make some syntax changes here and there due to GCC being much more strict than msvc. OpenGL declarations code is a mess but I cleaned up the linux part (no need for glext.h).

Unfortunately there is still a major bug left. Some (all?) effects are visible on top of entities and others don't show up at all. Same code runs ok on windows. I also tested ioquake/jedi-outcast on linux and it looks good. Some stencil buffer issues perhaps? I don't really know this part of the engine. Do you have any ideas what may be cause of this?
![shot 0004](https://cloud.githubusercontent.com/assets/4474909/4606782/f9da13dc-5230-11e4-9ee8-a10f2722a105.png)