The Makefile was hard-coded to some GNU/Linux specific commands and
arguments:
- `cp -u` does not exist on macOS (and possible BSD either).
Likely, the rsync is already used in other steps, so we can use it
in place of `co -u` as well.
- On macOS there is no sma256sum command.
Not by default, anyway. While it is probably available via installing
core utils via brew, this is quite big package which could cause other
issues, so it is not really no-brainer to install.
This is solved by detecting availability of sha256sum command and
using `shasum -a 256` if the former is not available.
