Link with pthread

[frederikaalund]

When importing uvloop, I got the following error:

>>> import uvloop
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/projects/RedPitaya/OS/buildroot/buildroot-2017.02/output/target/usr/lib/python3.5/site-packages/uvloop/__init__.py", line 7, in <module>
ImportError: /usr/lib/python3.5/site-packages/uvloop/loop.cpython-35m-arm-linux-gnueabihf.so: undefined symbol: pthread_atfork

This is fixed by linking the plugin (loop.cpython-35m-arm-linux-gnueabihf.so) to pthread as done in this patch.

[1st1]

Do we need to do this for all platforms or for Linux+arm only?

[frederikaalund]

Do we need to do this for all platforms or for Linux+arm only?

Good point. Theoretically, it's needed for all operating systems that use POSIX threads, which is most Unix-like operating systems (Linux, MacOS, etc.). Practically speaking, that is every platform except Windows.

I'm surprised that I'm the first person to get the undefined symbol error. Apparently, this error only triggers in the Linux + ARM combination? Anyhow, linking explicitly against pthread is the way to go.

Also, consider using -pthread instead of -lpthread. I know that I use -lpthread in my patch; I learnt about the difference after making the pull request.

[1st1]

Merged as is. Thanks!