We already had the pointer size check in the upstream order in our
genesis open source commit 6b0e52be , but we seem to have missed the
rest of the rearrangement from upstream.

This commit completes that cherry-pick, to reduce merge diff with
(admittedly much later) commit 46328916 .

> Doesn't seem like a good idea to be doing AC_CHECK_SIZEOF(void *) so much
> earlier than all the other sizeof checks, and it certainly fails to follow
> the order suggested at the file head.  Rearrange.
Author: Tom Lane
Adapted by Pivotal

(cherry picked from commit 075ac80d)