Provide pkg-config file

[tintou]

This allows to use the pkg-config mechanism on supported systems.

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[tintou]

@googlebot I signed it!

[Losergfsfyjg]

Delete

[tintou]

@mandlil Hi there, do you have a bit of time to review this PR?

[mandlil]

Hi there, thank you for your PR. We are apologize for late reply. Currently we don't have anyone in the team to review such C++ changes. Please note our contribution guidelines, where we are committing to review and accept PRs for documentation changes and bug fixes, but due to lack of staffing, we cannot promise anything else. See
our contribution guidelines:
https://github.com/google/libphonenumber/blob/master/CONTRIBUTING.md.