This routine destroys the condition variable specified by cond.
    This effectively uninitializes the condition variable. Call this
    routine when a condition variable will no longer be referenced.
    Destroying a condition variable allows the Threads Library to
    reclaim internal memory associated with the condition variable.

    It is safe to destroy an initialized condition variable upon
    which no threads are currently blocked. Attempting to destroy a
    condition variable upon which other threads are blocked results
    in unpredictable behavior.

    The results of this routine are unpredictable if the condition
    variable specified in cond either does not exist or is not
    initialized.