Fix broken ifndefs
    
    ifndef doesn't work for those functions that aren't already macro definitions in
    tevent.h like tevent_req_data. However if tevent_req_data is defined in
    tevent.h, all other functions are available as well. So just check for
    tevent_req_data and define all of them if it is not available.
    This should fix compiling on distros with the latsest tevent.