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.