[Bug c++/93821] Define __cplusplus to 202002L in C++20

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93821

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
No, because that just makes it more awkward to write makefiles and build
scripts. We used to do that, and it had no real benefit. If you want to use
c++2a, you can use that. Other people just want to use c++20 across multiple
releases, some of which have full C++20 support and some don't.

To check the level of conformance you should check the macros defined by the
compiler when compiling, not rely on which command-line options happen to be
supported.