I found this discussion:
"Newer versions of the MySQL client distributions do not include the "_r" version"
and
"libmysqlclient_r.a is "re-entrant". .. But as .. pointed out, there is no difference in newer versions (both are re-entrant).
However .. the codebase for libmariadb is the 3.23 libmysql (what was LGPL licensed - later versions were not and are not). So this could be something added in libmysql that libmariadb did not consider and this could be the reason. But we (Webyog) have no problems with coding multithreaded applications using libmariadb. But I am not a programmer, so I cannot comment further.
This belongs to Georg Richter!
-- Peter
-- Webyog