mysql: fix debugging options

The usual --{en,dis}able-debug passed by the infrastructure are not
recognised by mysql's ./configure, as it uses unusual options.

Also, debugging is only meaningful to the server, not the client.

Signed-off-by: Marcelo Gutierrez (UTN/FRH) <kuyurix@gmail.com>
[yann.morin.1998@free.fr: actually add the options, move in server if-block]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>