Merge stdenv-updates into master

This is a re-merge, in fact. Now the first parent is master
as of right before the "first merge" (ac6761c9).
A few commits pushed in-between into master now look like coming
from stdenv-updates, but otherwise this seems to fix the situation.

Cf. https://github.com/NixOS/nixpkgs/commit/320209a618221728e022222a2aafee1af72037bb#commitcomment-5125991