cmake HAVE_CXX17 examples

STEllAR-GROUP/hpx cmake/HPX_AddDefinitions.cmake :137 

string(FIND ${def} "HAVE_CXX17" _pos)

STEllAR-GROUP/hpx cmake/HPX_AddDefinitions.cmake :171 

string(FIND ${def} "HAVE_CXX17" _pos)