DO NOT MERGE PRs while there are still discussions
As Matthew said, the discussion was about foreach_, which the merged code no longer used.
neither of us nominated reviewers had approved.
It seemed Matthew agreed with the change because he suggested wording for the HACKING file. He hadn't commented on the build.c change, but as he appeared to support the principle in the HACKING file, the build.c change was enforcing the principle.
BTW, I'm not sure what you mean about 'nominated reviewers'?
I am tempted to revert on principle
You had told me a few months ago to leave pull requests open for at least a week, including a full weekend, before merging (I would still wait longer for any change likely to need discussion). I hadn't seen any new criteria for merging since then. Reverting should only be done if there's a commit that causes more harm than good, and the author is not able to fix it speedily.