Created by: luotao1

fix https://github.com/PaddlePaddle/Paddle/pull/9891#discussion_r181616470

• users can set -DTENSORRT_ROOT=xxx to specify the tensorrt library path.
• cmake would print messages like Current TensorRT header is /home/luotao02/tools/TensorRT-3.0.4/include/NvInfer.h. Current TensorRT version is v3.