Returns the (relative) path to the Boost source-directory this file is located in (if any).

def current_boost_dir():
    """Returns the (relative) path to the Boost source-directory this file is located in (if any)."""
    # Path to directory containing this script.
    path = os.path.dirname( os.path.realpath(__file__) )
    # Making sure it is located in "${boost-dir}/libs/mpl/preprocessed".
    for directory in reversed( ["libs", "mpl", "preprocessed"] ):
        (head, tail) = os.path.split(path)
        if tail == directory:
            path = head
        else:
            return None
    return os.path.relpath( path )