Allow using prebuilt libkvscproducer through pkg-config

[hacker1024]

Description of changes:

At the moment, the use of libkvscproducer does not respect -DBUILD_DEPENDENCIES. This PR allows it to be found via pkg-config instead of built from source.

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[sirknightj]

Hi @hacker1024, thanks for your contribution.

We want to avoid breaking backwards compatibility. The behavior we want to make sure doesn't get broken, is that when running cmake .. -DBUILD_DEPENDENCIES=OFF after a fresh git clone, it still builds C producer (for a new user who doesn't have C producer cloned or built). I would suggest adding a new flag which build producer-c defaulting to ON to maintain this behavior.

In your PR description, could you also add how you tested it (which scenarios/commands you ran)?

Finally, could you change the target branch to develop? We'll merge develop into master branch during the next release cycle.

Looking forward to these enhancements!