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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def compile_protos():
 """Builds necessary assets from sources."""

  # If there's no makefile, we're likely installing from an sdist,
  # so there's no need to compile the protos (they should be already
  # compiled).
  if not os.path.exists(os.path.join(THIS_DIRECTORY, "makefile.py")):
    return

  # Only compile protobufs if we're inside GRR source tree.
  subprocess.check_call(
      ["python", "makefile.py", "--clean"], cwd=THIS_DIRECTORY)