Hi,There isn't really a way to verify the thread safety aside from what's in the test suite, but how old systems do you consider when you say «old boxes»? Ones with eol'ed versions of MySQL (< 5.5)? In 5.5 and 5.6 there's no difference between libmysqlclient and libmysqlclient_r, as the latter was only included for backwards compatibility.
I can do some digging to find out exactly when _r was made a symlink.
-- Lars
