Document how to install CommonMark on newer Macs

[oalders]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.94%. Comparing base (896e21d) to head (64f2a46).

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[haarg]

I think this may be better left as part of the brew install carton command. The elaboration added in this PR doesn't change anything about how cmark is installed, just how it's used. And it should generally be installed before running carton.

Linking against the version specific paths shouldn't be needed and will be fragile. Upgrades could lead to breaking your build. So I'd rather list it as a fallback in case the normal path didn't work rather than the expected path.

[haarg]

So I've figured out what allowed this to work fine for me on my system. I have LIBRARY_PATH and CPATH set to allow compiling against the homebrew libraries.

LIBRARY_PATH=/opt/homebrew/lib CPATH=/opt/homebrew/include cpm install -g CommonMark is likely a better command to recommend.

[oalders]

@haarg this is ready for another review.