Let function(open3d_build_3rdparty_library name) look for .pc as well?

[jiapei100]

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Checklist

• [X] I have searched for similar issues.
• [X] For Python issues, I have tested with the latest development wheel.
• [X] I have checked the release documentation and the latest documentation (for master branch).

My Question

It looks function(open3d_build_3rdparty_library name) in find_dependencies.cmake ONLY searches for .cmake, rather than .pc.

Can anybody help???

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