Updating top-level autoconf to 2.59

[Paolo Bonzini]

>> What are they meant to be for build A, host B, target B?
> 
> Specify --build and --host.
> 
>> What are they meant to be for build A, host A, target A?
> 
> Specify --build.
> 
>> What are they meant to be for build A, host A, target B?
> 
> Specify --build, --target.

I agree that it would be best if we could just allow specifying all 
three and do the right thing in these cases too.

Would it be enough to override Autoconf's decisions on the program_prefix?

> 2.13 defaults build to host and runs config.guess if you do not specify
> host.  2.59 defaults host to build and runs config.guess if you do not
> specify build.  They are clearly incompatible, and the 2.59
> interpretation is pretty clearly more sensible, too.

That's fine.  The problems with Autoconf 2.59 stem from the fact that it 
defaults to cross compiling if --host is specified (which is not a 
problem, also because they have an "interim" measure where this is not 
done), and ...

> Autoconf 2.59 assumes you want a cross tool if you specify --target,
> no matter what you specify it as.

... this, which was likely implemented without any discussion with 
people really building cross tools.

Paolo