def _get_prepend(compiler, osname):
    """Return the appropriate prepend for a compiler switch for a OS.

    Parameters
    -------
    compiler : str
        compiler name
    osname : str
        lower case OS name

    Returns
    -------
    str : str
        prepend string for a compiler switch for a OS

    """
    if compiler in ["gfortran", "gcc", "g++", "clang"]:
        prepend = "-"
    else:
        if osname in ["linux", "darwin"]:
            prepend = "-"
        else:
            prepend = "/"
    return prepend