Weak pthread function

[H.J. Lu]

> 
> > libobjc.a uses pthread_key_create, pthread_create, pthread_setspecific,
> > thread_getspecific and pthread_mutex_trylock. We should provide them
> > in libc as weak functions.
> 
> Why doesn't libobjc.a use them with weak externs and handle their absence?

Good idea. Someone should fix libobjc.a.

H.J.