Mention pkg-config as a Linux build dependency

[jacobvosmaer]

Background in #10058

Without pkg-config, make fails with a generic error message pointing to INSTALL. But INSTALL does not clearly state pkg-config as a Linux build dependency.

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[grpc-kokoro]

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[grpc-kokoro]

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[grpc-kokoro]

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[grpc-kokoro]

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[grpc-kokoro]

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[grpc-kokoro]

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[grpc-kokoro]

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[grpc-kokoro]

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[jacobvosmaer]

I'm in talks with my employer on how they want me to handle the CLA.

[grpc-kokoro]

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[grpc-kokoro]

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[mehrdada]

@jacobvosmaer Can you please follow the @googlebot instructions above so it retries the CLA check on this pull request?

[jacobvosmaer]

I signed it!

(Using the literal words suggested by googlebot). More specifically, my employer GitLab Inc. signed the CLA recently.

Thanks for tht ping @mehrdada

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[mehrdada]

Thanks for the PR, but thinking about this a bit more today, I am reluctant to merge this change, since we don't really need pkg-config to accomplish the instructions in INSTALL.md (as I re-verified on a clean Debian 8 install just now) so I don't want us to instruct users to install any unnecessary dependencies that are not absolutely necessary.