Hi,

> There's no file on my machine which starts with libmysqlclient
> according to locate, mdfind and spotlight.

same here.

> And taking -arch i386 out of the compile command removes the i868 error
> line, but there's still:
> mysql.c: In function 'query':
> mysql.c:635: error: invalid storage class for function 'res_free'


It worked for me using 2.5.2
(http://www.tmtm.org/en/mysql/ruby/mysql-ruby-2.5.2.tar.gz) and
typing:

tar xzf mysql-ruby-2.5.2.tar.gz
cd mysql-ruby-2.5.2
ruby extconf.rb --with-mysql-dir=/usr/local/mysql
make
make install


Patrick