GH-53995: [C++] Fix building Arrow via FetchContent with older versions of CMake

[raulcd]

Rationale for this change

For older versions of CMake the variable arrow_SOURCE_DIR is overriden by FetchContent's one because project(arrow) sets it as a cache variable while FetchContent sets a normal variable with precedence. Arrow can't find thirdparty/versions.txt because it is pointing to the tarball root instead of the cpp subfolder. See policy https://cmake.org/cmake/help/latest/policy/CMP0180.html
This has been tested and found at Iceberg-cpp here:

• build: apply patch on Arrow so it can be used with older versions of CMake iceberg-cpp#1029

What changes are included in this PR?

Use ARROW_SOURCE_DIR instead of arrow_SOURCE_DIR so FetchContent's arrow_SOURCE_DIR does not have precedence.

Are these changes tested?

Yes locally on the iceberg PR on Ubuntu 24.04.

Are there any user-facing changes?

No

Was AI used for this PR?

In accordance to the AI generation guidelines, please disclose below whether and how AI was used in this PR.

PR code and description written by:

• Human
• AI

I did use AI to initially find the root cause.

Reviewed before submission by:

• Human
• AI
• Not reviewed

• GitHub Issue: [C++] Building Arrow via FetchContent can fail with versions older than CMake 3.30 #53995

[github-actions]

⚠️ GitHub issue #53995 has been automatically assigned in GitHub to PR creator.

[Copilot]

🟢 Approval recommended

The narrowly scoped replacement uses an initialized source-directory variable and resolves the reported collision without identified regressions.

No open issues 🎉

What changed in this PR

Fixes Arrow C++ builds using FetchContent with older CMake versions by avoiding a source-directory variable collision.

Changes:

• Uses ARROW_SOURCE_DIR to locate third-party files under the C++ source directory.

File	Description
cpp/​cmake_modules/​ThirdpartyToolchain.cmake	Replaces the FetchContent-shadowed source-directory variable.

_{Review effort: Balanced}