Avoid #include "windows.h", just relevant parts (like GLFW3)
I want to use glad in my project ([raylib](https://github.com/raysan5/raylib)) but some symbols defined by `windows.h` conflict with my symbols. If I comment `#include "windows.h"` on `glad.h` everything compiles fine but programs crash on init.

Would it be possible to include only the required `windows.h` parts into `glad.h`? 

GLFW3 works [this way](http://www.glfw.org/docs/latest/build.html#build_include).
