Why no stdnoreturn?

[l0r3m1psum]

Premise: I'm fairly new to Github and C so I might be creating a useless issue and there may be an obvious reason for why you are doing so and I simply don't know it, that sad.

There are multiple instances in the code where __attribute__ ((noreturn)) is used instead of the noreturn macro defined in stdnoreturn.h, in file like xhyve.c, and as far as I know this is the standard way for making a function as non returning, so why not using it?

P.S.
I'm not a native speaker so forgive any error or any unclarity in my exposition.