code: def jarFlags(target, source, env, for_signature):
      
      jarflags = env.subst('$JARFLAGS', target=target, source=source)
      for src in source:
          contents = src.get_text_contents()
          if contents[:16] == "Manifest-Version":
              if not 'm' in jarflags:
                  return jarflags + 'm'
              break
      return jarflags
docs: If we have a manifest, make sure that the 'm'
      flag is specified.