cmake: Use -std=gnu++11 instead of -std=c++11

This fixes building with `-DSC_WII=ON` (which is ON by default).
Detail build error can be found [on the dev list](http://new-supercollider-mailing-lists-forums-use-these.2681727.n2.nabble.com/build-err-td7602863.html) (reported by @smrg-lm).

I have no way to test that on osx though..