Report the specific C++ standard at init time, stop warning about C++97/03

[giacomofiorin]

We are using enough C++11 features now that it would be impossible for anybody to build without it. On the other hand, we may be having a similar situation with newer C++ standards.

I'm proposing to just print out the raw value of __cplusplus at init time, while also warning that this might be inaccurate on Windows builds. Folks use VMD much more than other codes on Windows, but VMD never took in my patch to fix the reporting problem:

colvars/cmake/CMakeLists.txt

Lines 69 to 70 in 2d9ca52

	# Tell MSVC to actually report the C++ version
	target_compile_options(colvars PRIVATE $<$<CXX_COMPILER_ID:MSVC>:/Zc:__cplusplus>)

https://learn.microsoft.com/en-us/cpp/build/reference/zc-cplusplus?view=msvc-170