One more question:
What OSX are using??
Not being a great coder myself, it looks as though the file conftest is causing the issue. Unfortunately mysql-ruby may not be compilable on OSX. Did you try to compile it without the include and lib options set? Not knowing anything about mysql-ruby, is there a config file that needs configuring first? The mysql on OSX from Mark Lyniage is the same MySQL run on all other platforms except that it has been compiled for OSX. Did you try to build MySQL form souce? There may be a setting that needs to be set on the MySQL build before mysql-ruby will play nice. Try getting the source for mysql off of the entropy site and building from scratch. Also contact Mark Liyanage about there not being a mysqlclient library. He is very helpful. And since he has Ruby on his site, he may have already solved this issue.
Write back and let us know how it goes.
Best.
SA