Add CMAKE_CXX_STANDARD and CXX_STANDARD value of LATEST
A new CMAKE_CXX_STANDARD
value of LATEST
would apply the flag /std:c++latest
for MSVC. For compilers with no such equivalent, it would apply the highest -std=
value support by the compiler, as described at https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD.html.