Merge pull request #2118 from sparklemotion/flavorjones-update-depend…

…-file

chore: update depend file and provide a rake task to regenerate it

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**What problem is this PR intended to solve?**

When developing Nokogiri, having correct dependencies in the Makefile is helpful and necessary to a tight feedback loop.

The `ext/nokogiri/depend` file was generated over ten years ago by Nobu and I'd like to have a way to easily keep it up-to-date.

**Have you included adequate test coverage?**

Normal CI test suite should provide adequate coverage of compilation-during-installation.

**Does this change affect the C or the Java implementations?**

No functional changes for end users.