Use Pthreads locking and thread ID functions

Property definitions

libevent :: libevent $ Sys :: use_pthreads
# Use Pthreads locking and thread ID functions
fun use_pthreads: Bool `{
#ifdef EVTHREAD_USE_PTHREADS_IMPLEMENTED
	return evthread_use_pthreads();
#else
	return -1;
#endif
`}
lib/libevent/libevent.nit:588,1--595,2