The SNMP++ 3.4.0 release did not include an important refactoring of the name of the Snmp::get_v3mp() method to Snmp::get_mpv3() which is used by AGENT++ 4.3.0 and AgentX++ 2.2.0.
The SNMP++ 3.4.0a release fixes that, if you are intend to use SNMP++ 3.4.0 in conjunction with AGENT++/AgentX++ need should upgrade to 3.4.0a.