cmake: we need the gcc flag -march=auto even with -mavx512 flags on Skylake,
cmake: we need the gcc flag -march=auto even with -mavx512 flags on Skylake,

This probably breaks compilation on KNC, @te42kyfo do you think anyone will complain?
Haven't tested it on KNL either, but I think I'm the only fool who tries using GCC on that platform.

RRZE-HPC/GHOST,RRZE-HPC/GHOST,RRZE-HPC/GHOST,RRZE-HPC/GHOST