This routine destroys the condition variable specified by cond.
    After this routine is called, the Threads Library may reclaim
    internal storage used by the condition variable object. Call this
    routine when a condition variable will no longer be referenced.

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

    For more information about actions when threads are present,
    refer to the pthread_cond_destroy() description.