[Bugfix][CI/Build] Fix build error when using QUTLASS_SRC_DIR to build vllm from source.

[FENP]

Purpose

#24440 brings in the QuTLASS library, and I found a small build issue here.
A FATAL_ERROR will be triggered when using QUTLASS_SRC_DIR to build vllm from source.

vllm/cmake/external_projects/qutlass.cmake

Lines 29 to 31 in 98f30b8

	if(NOT qutlass_SOURCE_DIR)
	message(FATAL_ERROR "[QUTLASS] source directory could not be resolved.")
	endif()

This pull request fixes build error when use local qutlass source directory via environment variable.

Test Plan

QUTLASS_SRC_DIR=/path/to/qutlass uv pip install -e .

Test Result

Build on SM90 and everything is ok (qutlass build is skipped).

cc @LopezCastroRoberto @mgoin

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request correctly identifies and fixes a build error that occurs when building with a local QUTLASS_SRC_DIR. The issue is that qutlass_SOURCE_DIR was not being set in this scenario, leading to a fatal error. Your change directly addresses this by setting the variable. I've provided one suggestion to use a more idiomatic FetchContent command, which also solves the issue while improving consistency within the script.

[LopezCastroRoberto]

LGTM! Thanks for the fix.

[FENP]

#26773 was merged to fix the error. Close