document libANN build dependency

Matthew Fernandezrequested to merge
smattr/graphviz.gitlab.io:smattr/forum-3188 into main

Forum: https://forum.graphviz.org/t/licence-for-libann-is-gpl-and-not-lgpl/3188
Reported-by: Robert Hairgrove

This also addresses some inconsistencies in macOS documentation. Though as I note in one of the commits:

I would say the Macports instructions here should probably be considered harmful. They discuss an older version of macOS and I do not know why they instruct you to first uninstall Macports. It is also unclear to me why the first code block lapses into English prose as if these are valid commands to run.

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