MySQL on Debian, unknown variable ‘set-variable=local-infile=0’

I have a dedicated server which is running MySQL.

In the my.cnf configuration file, I added this configuration a long time ago :

set-variable=local-infile=0

It was working fine, but since yesterday, my server started to act strangely, and after a reboot, I was unable to start mysql.

The probleme was located at this configuration, as stated in the logs :

[ERROR] /usr/sbin/mysqld: unknown variable 'set-variable=local-infile=0'

And after having commented this line, mysql started fine.

Is there something I missed, an update that break this type of configuration ? I'm really wondering what happened.

I run MySQL 5.5.31-0+wheezy1

(Note: I'm aware of this quite similar question, but I'm looking to understand what changed, not where is the problem ;))

Thanks for your help!