call __signbit for CUDA >= 6.5 implementation

src/caffe/util/math_functions.cu

@@ -329,7 +329,12 @@ void caffe_gpu_powx<double>(const int N, const double* a,
 
 DEFINE_AND_INSTANTIATE_GPU_UNARY_FUNC(sign, y[index] = (Dtype(0) < x[index])
                                       - (x[index] < Dtype(0)));
+#if CUDA_VERSION >= 6050
+// __signbit to pick up the CUDA function.
+DEFINE_AND_INSTANTIATE_GPU_UNARY_FUNC(sgnbit, y[index] = __signbit(x[index]));
+#else
 DEFINE_AND_INSTANTIATE_GPU_UNARY_FUNC(sgnbit, y[index] = signbit(x[index]));
+#endif
 
 __global__ void popc_kernel(const int n, const float* a,
     const float* b, uint8_t* y) {