MT-safe exception and thread library

[H.J. Lu]

> 
> Does the following make any sense?  It should combine the existing
> functionality to allow compiling with -D_PTHREADS etc, or if not
> defined, uses the --enable-threads and weak symbols.
> 

I will be gone for 3 weeks. I will try it when I come back.

Thanks.

H.J.