Only read from MYSQL_HOME if MARIADB_HOME is not set

After CONC-449 MariaDB Connector/C is loading files from the directory MYSQL_HOME and MARIADB_HOME.
This implementation should be changed to first check if MARIADB_HOME is set and if it is, use only this directory for loading configuration files. Only if MARIADB_HOME is not set, use the home directory as set by MYSQL_HOME.

This way we are still compatible with MySQL but can set the home directory for MariaDB also by MARIADB_HOME. This also allows to use different home directories if both MySQL and MariaDB Connectors are installed