add C++26 version/header marcos

[gpeterhoff]

Please add support for C++26 in boost/config/detail. I hope it is not too late for 1.89.

cxx_composite.hpp

#if defined(BOOST_NO_CXX23)\
   || defined(BOOST_NO_CXX26_HDR_CONTRACTS)\
   || defined(BOOST_NO_CXX26_HDR_DEBUGGING)\
   || defined(BOOST_NO_CXX26_HDR_HAZARD_POINTER)\
   || defined(BOOST_NO_CXX26_HDR_HIVE)\
   || defined(BOOST_NO_CXX26_HDR_INPLACE_VECTOR)\
   || defined(BOOST_NO_CXX26_HDR_LINALG)\
   || defined(BOOST_NO_CXX26_HDR_RCU)\
   || defined(BOOST_NO_CXX26_HDR_SIMD)\
   || defined(BOOST_NO_CXX26_HDR_TEXT_ENCODING)
#    define BOOST_NO_CXX26
#endif

suffix.hpp

#if (!defined(__has_include) || (BOOST_CXX_VERSION < 202400L))
#  define BOOST_NO_CXX26_HDR_CONTRACTS
#  define BOOST_NO_CXX26_HDR_DEBUGGING
#  define BOOST_NO_CXX26_HDR_HAZARD_POINTER
#  define BOOST_NO_CXX26_HDR_HIVE
#  define BOOST_NO_CXX26_HDR_INPLACE_VECTOR
#  define BOOST_NO_CXX26_HDR_LINALG
#  define BOOST_NO_CXX26_HDR_RCU
#  define BOOST_NO_CXX26_HDR_SIMD
#  define BOOST_NO_CXX26_HDR_TEXT_ENCODING
#else
#if (!__has_include(<contracts>) || !defined(__cpp_lib_contracts) || (__cpp_lib_contracts < 202502L)) && !defined(BOOST_NO_CXX26_HDR_CONTRACTS)
#  define BOOST_NO_CXX26_HDR_CONTRACTS
#endif
#if (!__has_include(<debugging>) || !defined(__cpp_lib_debugging) || (__cpp_lib_debugging < 202311L)) && !defined(BOOST_NO_CXX26_HDR_DEBUGGING)
#  define BOOST_NO_CXX26_HDR_DEBUGGING
#endif
#if (!__has_include(<hazard_pointer>) || !defined(__cpp_lib_hazard_pointer) || (__cpp_lib_hazard_pointer < 202306L)) && !defined(BOOST_NO_CXX26_HDR_HAZARD_POINTER)
#  define BOOST_NO_CXX26_HDR_HAZARD_POINTER
#endif
#if (!__has_include(<hive>) || !defined(__cpp_lib_hive  ) || (__cpp_lib_hive  < 202502L)) && !defined(BOOST_NO_CXX26_HDR_HIVE)
#  define BOOST_NO_CXX26_HDR_HIVE
#endif
#if (!__has_include(<inplace_vector>) || !defined(__cpp_lib_inplace_vector  ) || (__cpp_lib_inplace_vector < 202406L)) && !defined(BOOST_NO_CXX26_HDR_INPLACE_VECTOR)
#  define BOOST_NO_CXX26_HDR_INPLACE_VECTOR
#endif
#if (!__has_include(<linalg>) || !defined(__cpp_lib_linalg) || (__cpp_lib_linalg < 202311L)) && !defined(BOOST_NO_CXX26_HDR_LINALG)
#  define BOOST_NO_CXX26_HDR_LINALG
#endif
#if (!__has_include(<rcu>) || !defined(__cpp_lib_rcu) || (__cpp_lib_rcu < 202306L)) && !defined(BOOST_NO_CXX26_HDR_RCU)
#  define BOOST_NO_CXX26_HDR_RCU
#endif
#if (!__has_include(<simd>) || !defined(__cpp_lib_simd) || (__cpp_lib_simd < 202411L)) && !defined(BOOST_NO_CXX26_HDR_SIMD)
#  define BOOST_NO_CXX26_HDR_SIMD
#endif
#if (!__has_include(<text_encoding>) || !defined(__cpp_lib_text_encoding) || (__cpp_lib_text_encoding < 202306L)) && !defined(BOOST_NO_CXX26_HDR_TEXT_ENCODING)
#  define BOOST_NO_CXX26_HDR_TEXT_ENCODING
#endif
#endif

thx
Gero