title: python3 openCV install error: Symbol not found: _clock_gettime
body: <p>Can anyone help me fixing an error when importing opencv in python3?</p>
  
  <p>when I import cv2 in python3, I get something like</p>
  
  <pre><code>Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in &lt;module&gt;
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-
  packages/cv2/__init__.py", line 4, in &lt;module&gt;
  from .cv2 import *
  
  ImportError: dlopen(/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/cv2/cv2.cpython-36m-darwin.so, 2): 
  Symbol not found: _clock_gettime
  Referenced from: /Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/cv2/.dylibs/libavutil.55.78.100.dylib 
  (which was built for Mac OS X 10.12)
  Expected in: /usr/lib/libSystem.B.dylib
  in /Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/cv2/.dylibs/libavutil.55.78.100.dylib
  </code></pre>
  
  <p>Also, I'm using python version 3.6.3</p>