--enable-threadsafe default

We default to --enable-threadsafe.  If we're going that way with the
release, then at the very least we *must* document which OSes are known to
work, and which are known not to.  I think the answer to date is: Linux
yea, all else nae.  

If we can compile the list, perhaps we can autodetect the OS and adjust
the default for  --enable-threadsafe=[yes|no] accordingly.  

Are you using Linux and can't --enable-threadsafe?

Are you using --enable-threadsafe with anything else?  

(If some kind person wants to compile this list, I'd like that very much.)

Thanks.

--jkl