in macos CI run, build PNG from source and cache it instead of getting it from homebrew, then revert to using find_package() in CMake

[edwardhartnett]

See #164 (review)

[edwardhartnett]

This should solve #219.

[edwardhartnett]

The answer that worked was from here: torch/image#137

Basically, there is a libpng that is already installed on macos and is in a /Library/Framework directory which (on MacOS) cmake searches first. So it finds the older version of PNG and gets confused.

The answer is to delete that part of the /Library/Framwork directories like this:
sudo rm -rf /Library/Frameworks/Mono.framework