Use constexpr constants instead of defines.

[Klaim]

This is both to avoid using the preprocessor in C++ in general, and to prepare for future usage of the library as a C++ module.

[jjerphan]

How about using constinit?

[Klaim]

@jjerphan

How about using constinit?

constinit is useful for runtime static/global/namespace objects to guarantee that their initialization is compile-time, but they are neither const nor compile-time themselves.

The version number needs to be both compile-time and runtime and be kept const (bascially immutable), so the only option is constexpr.

constinit can be confusing, it's basically only useful to help with the "static init fiasco" issue.