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

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

It looks **function(open3d_build_3rdparty_library name)** in [find_dependencies.cmake](https://github.com/isl-org/Open3D/blob/master/3rdparty/find_dependencies.cmake) **ONLY** searches for **.cmake**, rather than **.pc**.

Can anybody help???

cheers