Initial report was:

The mysys library changed in MySQL 5.7, and as a result audit-plugin cannot find some symbols.

mysql> install plugin server_audit soname 'server_audit.so';

ERROR 1126 (HY000): Can't open shared library '/home/hf/mysql57/lib/plugin/server_audit.so' (errno: 2 /home/hf/mysql57/lib/plugin/server_audit.so: undefined symbol: _my_thread_var)

that was fixed, but the bigger problem is that API was radically changed in MySQL 5.7 so the plugin built on MariaDB cannot load there.
We need to decide something on it.