GCC-3.0.4 (pre) on AIX requires pthreads

[David Edelsohn]

	Thanks for the patch.  This may be necessary for the general
solution, but I think the AIX problem could be solved with something less
intrusive. 

	For AIX, if I could rip out the THREAD_MODEL_SPEC, uniformly
return 'aix' as the model, have libstdc++ install gthr-aix.h and its
dependencies, I think everything would be solved.  Using gthr-aix.h makes
each multilib thread header look identical once again, so it does not
matter which multilib directory actually succeeds in providing the
contents of include/g++-v3.

Thanks, David