Support header-file-only library generation
Would it be possible to add support for header-file-only generation for GLAD? I mean, similar to the [stb libraries](https://github.com/nothings/stb).

It would be very useful to just drop glad.h into the project and avoid pipeline modification.

[Here](https://github.com/raysan5/raylib/blob/develop/src/external/glad.h) it is a sample I'm working on; I created the GLAD header-file-only myself manually.
