Enable multi-core link time optimization

[roehling]

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

GCC runs LTO on a single core by default, making the final build step for the Python module needlessly long.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

The PR sets the optional auto argument for the -flto option, which will distribute the workload among the available jobs from GNU make (or, as fallback if GNU make is not used, the number of detected CPU cores).

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This change is 

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[ssheorey]

Hi @roehling Is this something we can have for the Open3D DSO as well? (in cpp/open3d/CMakeLists.txt)

[roehling]

Yes, but only after #4747 has been resolved.