to enable both recompiling mysqli or odbc from sources in addition to drop-in replacement functionality.

The case in question is compiling mysqli from sources, that needs client_errors via ER() macro.

Previously, we exported it as mysql_client_errors (compatibly to Fedora's style symbol renaming, see MDEV-3842).
However, if MariaDB header files are used when compiling mysqli, client_errors needs to be exported with its original name.