Several changes have happened in MariaDB 10.5, most notably:
* Information Schema table definitions have changed
* More things use LEX_CSTRING
This fixes all the compile issues
I reformulate the messages.
Changed version in preprocessor conditions to avoid compilation
warnings in Debian 9.
Disabled sign-compare check for generated files in DML/DDL.
Legacy system vars with names infinidb_* was preserved for
backward compatibility and they will be used if
columnstore_use_legacy_vars variable is set.
Remove unused structure and plugin variable.