Preliminary support for Linux with the engine.
Requirements for this milestone include:

• A shader API that can run on Linux (even if it's running dx9 through mesa)
• The engine must be able to run and load maps. I do not expect sound.
• Any crashes musn't be related to the low-level libraries like tier0, vstdlib, tier1, etc. These must absolutely be stable in order to progress with porting.
• Inputsystem must work and be stable
• The CMake build system must work well and be easy to use
• The posix-support branch must build on Windows with CMake or VPC, and it must be just as stable as dev/master.
• The posix-support branch will be merged into dev once this milestone is reached.
• Tools like vrad, vvis, vbsp, bsppack should be ported to Linux because of how vital they are.