cmake: simplify the way we choose the C++ standard to use
cmake: simplify the way we choose the C++ standard to use

This will make it easier to try out C++14 features

lyrl/atria,lyrl/atria,shaunstanislaus/atria,manashmndl/atria,AbletonAG/atria,bowlofstew/atria,shaunstanislaus/atria,manashmndl/atria,AbletonAG/atria,lyrl/atria,manashmndl/atria,respu/atria,AbletonAG/atria,respu/atria,bowlofstew/atria,kod3r/atria,dodheim/atria,Ableton/atria,iankronquist/atria,iankronquist/atria,Ableton/atria,respu/atria,dodheim/atria,kod3r/atria,dodheim/atria,bowlofstew/atria,iankronquist/atria,Ableton/atria,kod3r/atria,shaunstanislaus/atria