disable some avx2 optimizations in ospcommon due to compile errors w/ gcc-6
disable some avx2 optimizations in ospcommon due to compile errors w/ gcc-6

MengjiaoH/ospray,wilsonCernWq/ospray,MengjiaoH/ospray,ospray/OSPRay,MengjiaoH/ospray,MengjiaoH/ospray,ospray/OSPRay,ospray/OSPRay,MengjiaoH/ospray,wilsonCernWq/ospray,ospray/OSPRay,wilsonCernWq/ospray