Updating top-level autoconf to 2.59

[Paolo Bonzini]

>>> 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.
> 
> Speak for yourself - I like this, despite the migration challenge.
> It's useful for uniformity of a set of cross tools, one of which may
> just happen to match $host.

Okay, I won't challenge you :-)  But whatever changes there might be, 
they ought to be documented well.

Paolo