cmake: disable SC_WII by default

- WII support is broken on osx>10.7 (and possibly on linux too)
- it won't build on linux with `-DSC_WII=ON`
- see https://github.com/supercollider/supercollider/pull/986

Signed-off-by: Yvan Volochine <yvan.volochine@gmail.com>
Merge pull request #987 from gusano/topic/disable_WII

cmake: disable SC_WII by default