[delta_p_delta_x]

This is a much better idea, actually. You're right that what I mentioned is a bad misuse of C++ features.

I initially did think of simply letting the build system handle it (CMake can do this excellently, as can Meson, probably) by simply choosing different source files for different target OSs, but when typing out the parent comment, I forgot about it in my train of thought.

Come to think of it, this is far more elegant.