message: ocaml: Pretty print builders.
diff: @@ -257,6 +257,9 @@ class Compiler(drake.Builder):
  self.__toolkit = toolkit
  super().__init__([self.__deps, self.__source], [self.__target])
  + def __str__(self):
  + return 'compilation of {}'.format(self.__target)
  +
  def execute(self):
  self.cmd('Compile {}'.format(self.__target),
  self.__toolkit._compile_command(self.__source, self.__target, self.__config),
  @@ -306,6 +309,9 @@ class Linker(drake.Builder):
  self.__library = library
  super().__init__(sources, [target])
  + def __str__(self):
  + return 'linkage of {}'.format(self.__target)
  +
  def execute(self):
  sources = oset(filter(lambda n: n.__class__ is not InterfaceObject, self.__sources))
  sources = self.__toolkit._dependencies_topological(sources)
  