/usr/lib/x86_64-linux-gnu/libmysqlclient_r.a is in libmariadb-dev-compat 3.0.3-1build1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libmariadb.a
/usr/lib/x86_64-linux-gnu/libmariadb.a