code: def cpp_flag(compiler):
      
      standards = ['-std=c++14', '-std=c++11', '-std=c++0x']
      for standard in standards:
          if has_flag(compiler, [standard]):
              return standard
      raise RuntimeError(
          'Unsupported compiler -- at least C++0x support '
          'is needed!'
      )
docs: Return the -std=c++[0x/11/14] compiler flag.
      The c++14 is preferred over c++0x/11 (when it is available).