Pass --build=<triplet> to native builds by default?

[Mark H Weaver]

It turns out that on ARM systems, the result of 'config.guess' depends
on the result of 'uname -m'.  In other words, details of the kernel (and
perhaps processor?) on the build machine will determine the triplet of
our builds, which in turn may affect what set of instructions is used.

Therefore, I think that on ARM systems (and perhaps _all_ systems), we
should consider passing --build=<triplet> to configure by default.

What do you think?

    Mark