config.h.in --> glfw_config.h.in

I currently build my code along with glfw and other third-party libraries.  There are multiple config.h files generated and in certain situations glfw ends up including the wrong one .  In my personal copy of GLFW, I have already renamed GLFW's config.h.in to glfw_config.h.in to disambiguate the filename.  Since I prefer to avoid diverging from upstream as much as possible and in the hope that this change is not controversial I am issuing a pull request for your consideration.  Thanks.