Returns true if running on windows, whether in cygwin or not.

def on_windows ():
    """ Returns true if running on windows, whether in cygwin or not.
    """
    if bjam.variable("NT"):
        return True

    elif bjam.variable("UNIX"):

        uname = bjam.variable("JAMUNAME")
        if uname and uname[0].startswith("CYGWIN"):
            return True

    return False