-DPLUGIN_ROCKSDB=YES leads to errors during build

-DPLUGIN_ROCKSDB=YES lead to an error in attempt to build static engine:

sql/libsql.a(sql_builtin.cc.o):(.data.rel+0x18): undefined reference to `builtin_maria_rocksdb_plugin'

collect2: error: ld returned 1 exit status

Workaround: say nothing about PLUGIN_ROCKSDB to cmake and it will build dynamic.