Cannot install InnoDB/XtraDB plugin (undefined symbol: wsrep_md5_update)

InnoDB/XtraDB plugins use some wsrep_md5_xxx() functions defined
under mysys_ssl/my_md5.cc and hence the related symbols remain
undefined.

MariaDB [(none)]> INSTALL PLUGIN innodb SONAME 'ha_xtradb.so';
ERROR 1126 (HY000): Can't open shared library /install/lib/plugin/ha_xtradb.so' (errno: 2, undefined symbol: wsrep_md5_update)