missing version macros

Hello,
I miss these macros in boost/config.hpp:
- BOOST_NO_CXX20/BOOST_NO_CXX23 (like BOOST_NO_CXX14, BOOST_NO_CXX17)
- BOOST_CXX17_CONSTEXPR/BOOST_CXX20_CONSTEXPR/BOOST_CXX23_CONSTEXPR (like BOOST_CXX14_CONSTEXPR)
- BOOST_CXX20_CONSTEVAL/BOOST_CXX23_CONSTEVAL

This could be done like this:
// C++17
#if defined(__cpp_constexpr)
#if (__cpp_constexpr < 201603L)
#define BOOST_NO_CXX17_CONSTEXPR
#endif
#else
#define BOOST_NO_CXX17_CONSTEXPR
#endif

#if defined(BOOST_NO_CXX17_CONSTEXPR)
#define BOOST_CXX17_CONSTEXPR
#else
#define BOOST_CXX17_CONSTEXPR constexpr
#endif

// C++20
#if (BOOST_CXX_VERSION < 202002L)
#define BOOST_NO_CXX20
#endif

#if defined(__cpp_constexpr)
#if (__cpp_constexpr < 202002L)
#define BOOST_NO_CXX20_CONSTEXPR
#endif
#else
#define BOOST_NO_CXX20_CONSTEXPR
#endif

#if defined(BOOST_NO_CXX20_CONSTEXPR)
#define BOOST_CXX20_CONSTEXPR
#else
#define BOOST_CXX20_CONSTEXPR constexpr
#endif

#if defined(__cpp_consteval)
#if (__cpp_consteval < 201811L)
#define BOOST_CXX20_NO_CONSTEVAL
#endif
#else
#define BOOST_CXX20_NO_CONSTEVAL
#endif

#if defined(BOOST_CXX20_NO_CONSTEVAL)
#define BOOST_CXX20_CONSTEVAL
#else
#define BOOST_CXX20_CONSTEVAL consteval
#endif

// C++23
#if (BOOST_CXX_VERSION < 202100L)
#define BOOST_NO_CXX23
#endif

#if defined(__cpp_constexpr)
#if (__cpp_constexpr < 202211L)
#define BOOST_NO_CXX23_CONSTEXPR
#endif
#else
#define BOOST_NO_CXX23_CONSTEXPR
#endif

#if defined(BOOST_NO_CXX23_CONSTEXPR)
#define BOOST_CXX23_CONSTEXPR
#else
#define BOOST_CXX23_CONSTEXPR constexpr
#endif

#if defined(__cpp_consteval)
#if (__cpp_consteval < 202211L)
#define BOOST_CXX23_NO_CONSTEVAL
#endif
#else
#define BOOST_CXX23_NO_CONSTEVAL
#endif

#if defined(BOOST_CXX23_NO_CONSTEVAL)
#define BOOST_CXX23_CONSTEVAL
#else
#define BOOST_CXX23_CONSTEVAL consteval
#endif

thx
Gero

Start in boost/config.hpp, where the existing C++14 and C++17 version and feature macros are defined. Compare the requested C++20 and C++23 macros with those existing definitions, then verify that the requested version, constexpr, and consteval macros are available with the intended compiler feature checks.

Written by the indexing model from the issue text.