Ticket 70 - Make tevent an optional component of nunc-stans.
    
    Bug Description:  We enable and support libevent and libtevent for event
    notifications. However, we only use libevent in production in our code.
    
    Fix Description:  This makes tevent an optional build dependency rather than
    requiring both. This aids in porting to other platforms as we aren't requiring
    excess dependencies.
    
    We disable this at configure time rather than removing it, as we don't want to
    just "delete" the work put in to develop this. This way, in the future if
    we want to revive this we can. However, if we don't see usage in the next 6
    months we should delete this code.
    
    https://pagure.io/nunc-stans/issue/70
    
    Author: wibrown
    
    Review by: nhosoi (Thanks!)