[toplevel] Overhaul detection of target compilers

[Paolo Bonzini]

> I think I can approve it for GCC, though you should hold off a few days 
> for (a) build-system maintainers to comment, and (b) src/ approval.

gcc and src must be in sync, so this may not be good for applying it to 
the gcc repository.  OTOH, this patch affects only combined trees and 
for those you always use gcc's toplevel configury.

I'm leaving for a week on July 31st.  When I come back (I do not want to 
be away just after I commit a patch that can cause build trouble in 
stage3!), if nobody tells me otherwise, I'll commit to gcc and src the 
attached patch, which removes the possibly controversial change to 
MAKEOVERRIDES.

Paolo
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