Ensure the existence of a symbolic link pointing to src named dst. Returns
    a boolean indicating whether the symlink already existed.

def ensure_symlink (src, dst):
    """Ensure the existence of a symbolic link pointing to src named dst. Returns
    a boolean indicating whether the symlink already existed.

    """
    try:
        os.symlink (src, dst)
    except OSError as e:
        if e.errno == 17: # EEXIST
            return True
        raise
    return False