Add WITH_THREADING define to libmosquitto compilation

[tengelmeier]

libmosquitto needs to be compiled with

WITH_THREADING=1

otherwise the pthread mutexes get stubbed out and there is a race condition between the mosquitto_loop_forever thread and the thread where you send new packets...
(Hope i didn't overlook it's already fixed...)

[jmesnil]

Thanks for the bug report, I forgot to commit this change in the xcode project settings.
Done in 25ea739

[dbburgess]

I believe I'm getting this race condition in the latest; it appears it was reverted in 0c0c013.

[jmesnil]

I've added it back. I did not realise I removed it when I upgraded the project settings.