Get target libraries to obtain thread information from GCC

[Benjamin Kosnik]

Thanks for doing this. It's been a big issue for months. In particular, 
the AIX thread model has to work (it's something bizarre, where command 
line options change the threading model). I'm not quite sure how that is 
handled, but to be honest even consolidating all the various configure bits
will be a big step forward. Have you thought about this case?

The libstdc++ parts are fine, please consider this my ok for branch and 
trunk.

-benjamin

> This patch cleans up the thread-model recognition code in all target
> libraries such that it obtains the thread information out of GCC,
> instead of having (and failing) to duplicate the logic used in
> gcc/configure.in.
> 
> Tested on i686-pc-linux-gnu.  Ok to install?  Ok for 3.0 branch?