This is valid code in C++20:

    if constexpr (std::endian::native == std::endian::big) {
        std::cout << "big-endian" << '\n';
    }
    else if constexpr (std::endian::native == std::endian::little) {
        std::cout << "little-endian"  << '\n';
    }
    else {
        std::cout << "mixed-endian"  << '\n';
    }

Doesn't solve everything, but it's saner even if what you're writing is C-style low-level code.