Convert internal lock-management to pthread

[federicosilvestri]

You can't use an internal locking-unlocking system, because you should disable interruptions during lock or unlock execution (it's not possible with CPU in user-mode). So you can include pthread and use a mutex. Let me know for a pull-request.

[rxi]

I'm not sure I understand what you mean. The user can use a pthread mutex with the library by using the log_set_lock() function: https://github.com/rxi/log.c#log_set_locklog_lockfn-fn