CMake: Switch to --std=gnu++11 to fix builds with POWER

CMake: Switch to --std=gnu++11 to fix builds with POWER

On POWER architectures (ppc64/ppc64le), the AltiVec SIMD instructions
overload bool with vector ints, causing builds to fail because vector
data cannot be cast to scalar data. However, this only happens on c++11
mode. In gnu+11 mode, the usage of AltiVec overload attributes is
contextual, so it doesn't cause builds to fail anymore.