MDEV-8969 groonga is compiled even with -DPLUGIN_MROONGA=NO
MDEV-8969 groonga is compiled even with -DPLUGIN_MROONGA=NO

first add mroonga plugin (MYSQL_ADD_PLUGIN) then add dependent
targets (add_subdirectory(groonga))

ollie314/server,natsys/mariadb_10.2,natsys/mariadb_10.2,flynn1973/mariadb-aix,flynn1973/mariadb-aix,flynn1973/mariadb-aix,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,natsys/mariadb_10.2,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,flynn1973/mariadb-aix,ollie314/server,natsys/mariadb_10.2,ollie314/server,ollie314/server,ollie314/server,ollie314/server,natsys/mariadb_10.2,ollie314/server,ollie314/server,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,natsys/mariadb_10.2,natsys/mariadb_10.2,flynn1973/mariadb-aix,flynn1973/mariadb-aix