[gcc] Toplevel bootstrap fixes, 4/4

[Kelley Cook]

>> I don't remember, but there are definitely some versions of GCC
>> where it did not work.  It's also a bleeding pain to have the
>> build rely on magic environment variables.
> 
> Well, in theory it could be the real supported way if the toplevel 
> were Autoconf 2.5x, because you could mark AR_FOR_TARGET and other 
> variables as "precious". But practice is not theory as always.

Speaking of which, I was under the impression that Nathanael had the
toplevel autoconf 2.5x switchover pretty much ready to go was just
awaiting all the projects in the combined tree to switch first.  I
think that requirement has been fulfilled.

If someone doesn't beat me to it, I'll look at this over Christmas break.