GH-46684: [C++] Fix Meson configuration issue on Windows

[WillAyd]

Rationale for this change

This fixes an issue with the Meson configuration on Windows

What changes are included in this PR?

Fix up incorrect variable name usage

Are these changes tested?

Yes

Are there any user-facing changes?

No

• GitHub Issue: [C++] Meson Configuration Broken on Windows #46684

[github-actions]

⚠️ GitHub issue #46684 has been automatically assigned in GitHub to PR creator.

[kou]

+1

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit 2aaea00.

There were 73 benchmark results with an error:

• Commit Run on arm64-t4g-2xlarge-linux at 2025-06-03 11:06:12Z
  • tpch (R) with engine=arrow, format=parquet, language=R, memory_map=False, query_id=TPCH-07, scale_factor=1
  • tpch (R) with engine=arrow, format=native, language=R, memory_map=False, query_id=TPCH-09, scale_factor=1
• and 71 more (see the report linked below)

There was 1 benchmark result indicating a performance regression:

• Commit Run on amd64-m5-4xlarge-linux at 2025-06-03 10:18:46Z
  • partitioned-dataset-filter (R) with dataset=dataset-taxi-parquet, language=R, query=vignette

The full Conbench report has more details. It also includes information about 3 possible false positives for unstable benchmarks that are known to sometimes produce them.