```
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 D3907, D3909 and D3910.
Includes name changes to the gitian documentation which are not part of
the original PR.