Features: Add support for C++ 17 language standard
Add C++ 17 to CMake's language standard infrastructure (we already support 98, 11, and 14). For now record only the cxx_std_17
meta-feature. GNU and Clang compilers already support -std=c++1z
and -std=gnu++1z
flags, so record these flags for relevant versions of those compilers.
Closes: #16468 (closed)