Hi Elena,
Thanks for replying!
The patch was provided by one of the aforementioned task's commentators
http://bugs.mysql.com/file.php?id=17400&bug_id=29327
I haven't tested it but it looks like a simplest one, so should work.
It adds just few more lines to cover both RedHat and Debian include paths (not sure about Debian though cause I am CentOS user)
The only thing is that it's strictly searching under /etc/mysql.d/ path for *.cnf (in CentOS). (Maybe someone wants to have it under a different dir?)
So probably looking for includedir in my.cnf is desirable. But I would stay with default as is, after all it's FHS compliant 
I don't mind if this patch will be included since that's the dir path I anyway planned to use and obviously my files would have *.cnf extension
Thanks!
p.s. found many reports about mysqld_multi include problem here and there, even on Percona forums 
I hope MariaDB can finally make it happen!
Do you want to provide (contribute) a patch which we could apply? It would probably go much faster this way.