cmake CXX_HAS_WARNING examples

InsightSoftwareConsortium/ITK CMake/ITKSetStandardCompilerFlags.cmake :46 

check_cxx_compiler_flag(
      ${flag}
      CXX_HAS_WARNING${flag_var}
    )

ANTsX/ANTs CMake/ITKSetStandardCompilerFlags.cmake :48 

check_cxx_compiler_flag(${flag} CXX_HAS_WARNING${flag_var})