Hi

Marc Liyanage (http://www.entropy.ch/) gave me the right strings for 
the MySQL installed with his installer:

--with-mysql-include=/usr/local/mysql/include 
--with-mysql-lib=/usr/local/mysql/lib

It's also possible to compile it with the mysql that is installed 
with the 10.2 Server (after uncommenting have_library('m'); 
have_library('z') in extconf.rb):

--with-mysql-include=/usr/include/mysql
--with-mysql-lib=/usr/lib/mysql

Thanks!

Beat H.