Feature/fix enableauthentraps store

fix missing call to snmp_store_needed()

The writeable scalars of system_mib and snmp_mib does not trigger
the snmp_store to save the value after commit.
The netsnmp_watcher_helper_handler() no call snmp_store_needed()
after commit. This may be not always needed,
but at least snmp_mib and system_mib needs this!

It too add the missing configure test for clock_gettime()