Clean the build environment.

def Clean(self):
    """Clean the build environment."""
    # os.unlink doesn't work effectively, use the shell to delete.
    if os.path.exists(args.build_dir):
      subprocess.call("rd /s /q %s" % args.build_dir, shell=True)
    if os.path.exists(args.output_dir):
      subprocess.call("rd /s /q %s" % args.output_dir, shell=True)

    os.makedirs(args.build_dir)
    os.makedirs(args.output_dir)

    # Create virtualenvs.
    if not args.virtualenv_64bit:
      subprocess.check_call([self.virtualenv_bin64, self.virtualenv64])
    if args.build_32 and not args.virtualenv_32bit:
      subprocess.check_call([self.virtualenv_bin32, self.virtualenv32])

    # Currently this should do nothing as we will already have a modern pip
    # installed, but we leave this here so if we get broken by pip again it's
    # just a simple case of searching for pip>=8.1.1 and adding an upper limit
    # cap in all those places.

    cmd = ["-m", "pip", "install"]
    if args.wheel_dir:
      cmd += ["--no-index", r"--find-links=file:///%s" % args.wheel_dir]

    subprocess.check_call([self.virtualenv_python64] + cmd +
                          ["--upgrade", "pip>=8.1.1"])
    if args.build_32:
      subprocess.check_call([self.virtualenv_python32] + cmd +
                            ["--upgrade", "pip>=8.1.1"])
    os.environ["PROTOC"] = args.protoc