cmake HAVE_M_CXX_ examples

WohlSoft/SDL-Mixer-X cmake/mixerx_macros.cmake :33

check_cxx_compiler_flag("${OPTFLAG}" HAVE_M_CXX_${OPT_VAR})

TheXTech/TheXTech cmake/build_props.cmake :81

check_cxx_compiler_flag("${OPTFLAG}" HAVE_M_CXX_${OPT_VAR})

WohlSoft/Moondust-Project _common/build_props.cmake :78

check_cxx_compiler_flag("${OPTFLAG}" HAVE_M_CXX_${OPT_VAR})