Fix OpenCV compilation. maxpool_layer on GPU uses cuDNN.

src/darknet.c

@@ -370,6 +370,8 @@ int main(int argc, char **argv)
     gpu_index = find_int_arg(argc, argv, "-i", 0);
     if(find_arg(argc, argv, "-nogpu")) {
         gpu_index = -1;
+        printf("\n Currently Darknet doesn't support -nogpu flag. If you want to use CPU - please compile Darknet with GPU=0 in the Makefile, or compile darknet_no_gpu.sln on Windows.\n");
+        exit(-1);
     }
 
 #ifndef GPU