Tweak-header, A New Approach to Build-Time Library Configuration, by Colby Pike aka vector-of-bool. Oct 4, 2020

program.config.hpp

// Tweak-header, A New Approach to Build-Time Library Configuration,
// by Colby Pike aka vector-of-bool. Oct 4, 2020
// https://vector-of-bool.github.io/2020/10/04/lib-configuration.html

#include <cstdlib>
#include <string>

namespace program { namespace config {

    namespace defaults
    {
        constexpr bool do_audit() noexcept
        {
            return false;
        }

        inline bool do_audit_rt() noexcept
        {
            auto ev = std::getenv("PROGRAM_TWEAK_AUDIT");
            return ev && std::string(ev) != "0";
        }
    }

    using namespace defaults;
}}

// __has_include() is part of the C++17 standard, g++ supports it since v5
// https://en.cppreference.com/w/cpp/compiler_support#cpp17

#ifdef __has_include
# if __has_include("program.tweak.hpp")
#  include "program.tweak.hpp"
# endif
#else
# pragma message("program.config.hpp: Note: Tweak header not supported.")
#endif

program.cpp

// Tweak-header, A New Approach to Build-Time Library Configuration,
// by Colby Pike aka vector-of-bool. Oct 4, 2020
// https://vector-of-bool.github.io/2020/10/04/lib-configuration.html

#include "program.config.hpp"
#include <iostream>

int main()
{
    std::cout << std::boolalpha <<
        "program: do_audit():" << program::config::do_audit() <<
        " do_audit_rt():"      << program::config::do_audit_rt() << '\n';
}

// g++ -Wall -std=c++11 -o program.exe program.cpp && program.exe

program.tweak.hpp

// Tweak-header, A New Approach to Build-Time Library Configuration,
// by Colby Pike aka vector-of-bool. Oct 4, 2020
// https://vector-of-bool.github.io/2020/10/04/lib-configuration.html

namespace program { namespace config {

    constexpr bool do_audit() noexcept
    {
        return true;
    }
}}