Fall back to -std=c++2a on compilers without -std=c++20

[bernardladenthin]

Problem

test/CMakeLists.txt compiles test20.cpp with a hardcoded -std=c++20. GCC 8 — still
the system compiler on RHEL 8 and its rebuilds — only implements C++20 under the
pre-release spelling and rejects the flag outright:

g++-8: error: unrecognized command line option '-std=c++20'; did you mean '-std=c++2a'?

The test suite therefore cannot be built with it at all, although the library itself
compiles fine.

Fix

Probe the flag instead of the CMake feature. CMAKE_CXX_COMPILE_FEATURES lists
cxx_std_20 on GCC 8, because CMake maps that feature onto -std=c++2a — so the feature
list cannot tell us which spelling the compiler accepts. check_cxx_compiler_flag can,
and -std=c++2a is used where -std=c++20 is unknown.

This keeps test20.cpp a real C++20 translation unit on those compilers, rather than
silently falling back to the default standard.

Testing

Ubuntu 20.04 (glibc 2.31, so the library builds there regardless of #104), with g++-8
8.4.0:

	Result
before	build fails at test20.cpp: unrecognized command line option '-std=c++20'
after	432 pass

Regression checks, all unchanged:

Compiler	Environment	Result
GCC 13.3	Ubuntu 24.04	432 pass
clang 18.1	Ubuntu 24.04	432 pass
MSVC 19.44	Windows, Ninja	387 pass

On MSVC the probe correctly reports failure — cl.exe only warns (D9002: ignoring unknown option), but CMake's built-in fail patterns catch that. The -std=c++2a
fallback is then discarded anyway, because subprocess_set_cxx_flags skips the standard
flag whenever SUBPROCESS_CXX_USE_STD_FLAG is false.