GLFW: linux compiles and links

* Mostly updates to CMakeLists.
* OS.cpp used "wglew.h", which needed to be preprocessed-out on *nix

Other stuff:
* FPSCamera uses RotateNow instead of RotateTarget, since this fixed "inertial" movement with my mouse. Was that intended? Is that for people who are using joysticks?
* Based on how FindSOIL.cmake works, other files need to use `#include "SOIL.h"` instead of `#include "SOIL/SOIL.h"`