context::set_language does not correctly set __cplusplus #263

I am using wave as library in a tool which preprocesses C++ code to obtain some information and then lint it.
This requires preprocessing the code with a proper value for __cplusplus, as the code might use different #if .. branches depending on the value.

I know the C++ standard of the project. But I can't set __cplusplus myself because it is a special value which wave ignores when using the context::remove_macro_definition or context::add_macro_definition functions. This is fine, since the public API context::set_language exists.

However, I believe this API does not behave as expected:

• boost::wave::language_support::support_cpp14 does not exists, thus one cannot set __cplusplus accordingly Enable setting language standard C++14 #275
• boost::wave::language_support::support_cpp17 sets __cplusplus to 199711L, aka C++98 Correctly set __cplusplus for chosen C++ standard #264
• boost::wave::language_support::support_cpp23 sets __cplusplus to 199711L, aka C++98 Correctly set __cplusplus for chosen C++ standard #264
• boost::wave::language_support::support_cpp14 boost::wave::language_support::support_cpp26 does not exists, thus one cannot set __cplusplus accordingly. To be fair, C++26 is quite new and this point is more for completeness sake.

On the other hand, boost::wave::language_support::support_cpp, boost::wave::language_support::support_cpp11 and boost::wave::language_support::support_cpp20 cause __cplusplus to be set to the expected value.

I believe wave should offer a dedicated boost::wave::language_support::support_cppXX value for each released C++ version and set __cplusplus accordingly for preprocessing according to the values documented here

If you agree with the request I am happy to create a contribution for that.