libstdc++-v3 won't build on AIX 4.1 any more (3.0 branch)

[David Edelsohn]

	Fine with me.

	What doesn't work on AIX 4.1?  fetch_and_add() does not exist?
There is an older OS-provided primitive, but probably not worth it.  We
don't even build a pthreads version of the library on AIX 4.1.

Thanks, David