cmake HAS_CPP17_FLAG examples

jupyter-xeus/xeus-cling CMakeLists.txt :138 

CHECK_CXX_COMPILER_FLAG("-std=c++17" HAS_CPP17_FLAG)

xtensor-stack/xsimd test/CMakeLists.txt :57 

CHECK_CXX_COMPILER_FLAG("-std=c++17" HAS_CPP17_FLAG)