[toplevel] Overhaul detection of target compilers

[Paolo Bonzini]

> I can't really convince myself that (1) this will actually happen, and
> (2) we'd support it anyway, but the existing logic seems to indicate
> that we tried to support it already.


The previous logic was that CC_FOR_TARGET had to be specified in the 
environment (manually) if it was not ${target_alias}-gcc.

>Either way, we're breaking crosses, so let's make fixing this a high
>priority please.
>  
>
I have already posted a patch that looks for gcc *as well as* cc.  Sorry 
if I was not clear in the other message [which I did not mean to make 
private], which however did refer to the patch at 
http://gcc.gnu.org/ml/gcc-patches/2005-08/msg00574.html

Paolo