This has been fixed in upstream master branch with the introduction of CMAKE variable INSTALL_LIBDIR_DEB in commit https://github.com/MariaDB/mariadb-connector-c/commit/eb64582b68eb627d70dffad8808c7b6d4cf11b27
I have tested it and works well. Will upload to Debian as part of 3.0.6 when it is released.
