Although wxWidgets doesn't use any C++20 features yet, still allow
compiling it in C++ 20 mode if wanted.
Update the helper macro for detecting available C++ version support from
https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
with just a minor change to de-TAB-ify it to pass our code style check
forbidding indenting with TABs.
Closes#22238.