DOCS: For translation on MacOS, get OpenSSL from Homebrew and set env var.

Merged Bitbucket Importer requested to merge bitbucket/merged-pr-572 into branch/default

Created originally on Bitbucket by cjrh (Caleb Hattingh)

Was already merged in Bitbucket before import, marked as merged by the import user

This PR updates the docs for building.

As mentioned in #2174 (closed), on MacOS it is now necessary to install OpenSSL via Homebrew (or download the src manually), and set PKG_CONFIG_PATH environment variable.

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