[Build] Fixes for CMake install

[ProExpertProg]

This fixes the CMake install logic such that using cmake --install directly is supported. Necessary to fix CMake-based user workflows.

It also fixes the error on ROCm if CMake is used outside the venv (common for IDE-run CMake).

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[LucasWilkinson]

just checking python setup.py build_ext --inplace is not broken by this right?

[ProExpertProg]

just checking python setup.py build_ext --inplace is not broken by this right?

I tried pip install, but let me try a fresh python setup.py build_ext --inplace as well

[LucasWilkinson]

just checking python setup.py build_ext --inplace is not broken by this right?

I tried pip install, but let me try a fresh python setup.py build_ext --inplace as well

Please test fresh and incremental

[ProExpertProg]

Yep, tested all three :)

[LucasWilkinson]

Seems reasonable, will hold off till after the v0.9.0 release though to avoid anymore cmake thrash (bit of a battle recently haha)

[mgoin]

Are these "cmake user workflows" actually documented somewhere?

[ProExpertProg]

Are these "cmake user workflows" actually documented somewhere?

No but they should be, I can try to add something.