Disable platform guessing instead of adding dependencies-prefix

[radarhere]

Suggestion for python-pillow#8497

Rather than adding a new setting to setup.py, this just makes use of disable_platform_guessing to not add macOS paths.

I realise that you expect the path supplied to dependencies-prefix to be necessary, but looking at the output of the build, everything looks fine.

[freakboy3742]

Yes, the aim was to put dependencies-prefix into the path for linking purposes; but the pyproject.toml configuration of the PATH also achieves that.

There is a slightly ulterior motive because of a requirement on the next piece of work adding iOS support; but given there's already a setting related to this use case, I can go back to the drawing board on that one.