These headers have no associated C files, and are mainly used for
utility logic.
Having them in `/include` makes them more easily located, and more
globally used.
We are using "classic" header guards here over `#pragma once` for better
support. Since `#pragma once` is defined by the compiler, rather than
standard C.
Why isn't it used across the project? I have no idea.
this is the most portable and clean approach. Compiler built-ins should
always be present regardless, and is more maintainable, since we're not
trying to keep up with various versions of this header.
