Fix ISPC selection for MSVC15
Fix ISPC selection for MSVC15

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