Patch to update top-level config to autoconf 2.59

[Ben Elliston]

> > Here is my final patch to update autoconf in the src tree.  All subtrees
> > have been updated to be configured by autoconf 2.59 and this patch
> > updates the top level configure and Makefiles to also use 2.59.  The
> > only real change needed was to rename AS_FOR_TARGET to GAS_FOR_TARGET.
> > I also changed COMPILER_AS_FOR_TARGET to COMPILER_GAS_FOR_TARGET, though
> > that wasn't strictly necessary, it just seemed like a good idea to
> > maintain the naming consistency.
> 
> Why was the AS_FOR_TARGET change necessary?

I wondered that, too.  I'm concerned that this is a user-visible
change that might break users and/or build scripts.  It's possible to
use other assemblers for assembling target code and we should not
preclude this (or hint at it by virtue of its name).

Ben