cmake C_HAS_WARNING examples

InsightSoftwareConsortium/ITK CMake/ITKSetStandardCompilerFlags.cmake :30 

check_c_compiler_flag(
      ${flag}
      C_HAS_WARNING${flag_var}
    )

ANTsX/ANTs CMake/ITKSetStandardCompilerFlags.cmake :34 

check_c_compiler_flag(${flag} C_HAS_WARNING${flag_var})