cmake HAVE_CXX20 examples

STEllAR-GROUP/hpx cmake/HPX_AddDefinitions.cmake :130 

string(FIND ${def} "HAVE_CXX20" _pos)

STEllAR-GROUP/hpx cmake/HPX_AddDefinitions.cmake :164 

string(FIND ${def} "HAVE_CXX20" _pos)