Since 2016, with macOS 10.12 Sierra, Mac OS X has been renamed in macOS. It would be a nice if Bitcoin's macOS build instructions follow this naming convention to avoid misunderstandings.
Backport of core PR13366
https://github.com/bitcoin/bitcoin/pull/13366/files
Depends on D3910.
Includes name changes to the gitian documentation which are not part of
the original PR.