title: Psycopg2 error: Symbol not found _PQbackendPID
body: <p>I'm getting the following error from psycopg2:</p>
  
  <pre><code>Traceback (most recent call last):   File "test1.py", line 1, in
  &lt;module&gt;
      import psycopg2   File "/Users/xxx/Library/Python/2.7/lib/python/site-packages/psycopg2/__init__.py",
  line 50, in &lt;module&gt;
      from psycopg2._psycopg import (                     # noqa ImportError:
  dlopen(/Users/xxx/Library/Python/2.7/lib/python/site-packages/psycopg2/_psycopg.so,
  2): Symbol not found: _PQbackendPID   Referenced from:
  /Users/xxx/Library/Python/2.7/lib/python/site-packages/psycopg2/_psycopg.so
  Expected in: flat namespace  in
  /Users/eyabadal/Library/Python/2.7/lib/python/site-packages/psycopg2/_psycopg.so
  </code></pre>
  
  <p>Any suggestions on how to fix this?</p>