Load 'libc.dylib' or 'libc.so.6' depending on OS

[peterleiser]

Resolves this issue on Mac OS X v10.15.5:

➜ pymsgq git:(master) ✗ python3
Python 3.7.7 (default, Mar 10 2020, 15:43:33)
[Clang 11.0.0 (clang-1100.0.33.17)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

import msgq
Traceback (most recent call last):
File "", line 1, in
File "/usr/local/lib/python3.7/site-packages/msgq.py", line 26, in
libc=ctypes.CDLL('libc.so.6',use_errno=True)
File "/usr/local/Cellar/python/3.7.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/ctypes/init.py", line 364, in init
self._handle = _dlopen(self._name, mode)
OSError: dlopen(libc.so.6, 6): image not found