@Suiseiseki You can compile linux unmodified with the full LLVM/Clang toolchain, that's been a thing for quite a while now.
And gpl-2 doesn't means a lack of portability.

>I look for anything slightly GNU so I can add the GNU/.

I'd do almost the opposite, like how I'd almost call GNU Emacs, Gosling Emacs instead (specially as Stalledman often says he wrote Emacs, which he didn't).

>BusyBox/Linux
Nack, -linux-musl vs. linux-gnu* is the meaningful one, for binaries that is, x86_64-pc-linux-gnu is my Wintel.

>systemd/Linux
You forgot X11 :)