* Update macOS PATH shenanigans.
c.f., https://github.com/koreader/koreader-base/pull/1052
* Prefer coreutils, it now ships everything we need.
(Or it always did? I apparently don't actually have coreutils
installed on my end).
* Also recommend putting Homebrew first in PATH
Mainly because it turns out that's how I tested this the last time ;).
Also, it just makes sense, and might be part of Homebrew's
recommendations anyway.
* Made Porting not so hilariously outdated.
* Minor tweaks to the macOS build instructions and the various quirks involved in getting it to behave (and accompanying actual build fixes).
