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=gnu++1z flags, so record these flags for relevant versions of those compilers.
Closes: #16468 (closed)