solve the bug which the python include config gives out repeat results (#3742)

```
./configure 
Please specify the location of python. [Default is /home/wenjian/anaconda3/bin/python]: 
Do you wish to build TensorFlow with Google Cloud Platform support? [y/N] N
No Google Cloud Platform support will be enabled for TensorFlow
Found possible Python library paths:
  /home/wenjian/anaconda3/lib/python3.5/site-packages
  /home/wenjian/anaconda3/lib/python3.5/site-packages
Please input the desired Python library path to use.  Default is [/home/wenjian/anaconda3/lib/python3.5/site-packages]
```

it shows repeat result in config on python includes, it is because the old code show paths[0] and ret_paths at the same time,
so I modify the condition and do some simple optimization by using `join`