Add headless mode

[JohnnyCena123]

this is the initial implementation.
it might not be the cleanest, barely held together, but i can still polish it more
heres what i tested:

• building works for both make and cmake, examples are disabled entirely in headless mode
• rexm links with the built headless-mode raylib library, and works well
  and that's it really.

the main goal of this was to avoid linking graphical/audio/input related modules to cli tools which dont need it (i.e. rexm)

[JohnnyCena123]

@raysan5 I know you said you dont need it right now, but it can wait however much you want

[raysan5]

@JohnnyCena123 Thanks but there are no plans to add this change for now and keeping open PRs just generates burden to me. I prefer to avoid it if there is no intention to at least review (and potentially merge).