After reading
https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.70/autoconf.html#Specifying-Target-Triplets,
my understanding is that this change should mostly be a no-op, as
--build defaults to the output of config.guess, however, this may be
slightly more correct
>  For historical reasons, whenever you specify --host, be sure to
> specify --build too; this will be fixed in the future.

and will quell some warnings in depends (#16354). If anything, this
also explicitly enables cross-compilation mode when `--host` differs
from `--build`.

As for "fixed in the future", this is the case for Autoconf 2.70+.