GH-37107: [C++] Suppress an unused variable warning with GCC 7

[kou]

Rationale for this change

If there is a warning, debug build is failed by default.

What changes are included in this PR?

Suppress the unused warning explicitly.

Are these changes tested?

No.

Are there any user-facing changes?

Yes.

• Closes: [C++] Compilation error [building from source] in Arrow C++ debug build due to unused variable #37107

[kou]

@c0dzilla Could you confirm this with GCC 7?

[github-actions]

⚠️ GitHub issue #37107 has been automatically assigned in GitHub to PR creator.

[c0dzilla]

@kou confirmed that this works on g++ 7.3:

codzilla@hogwarts:~/arrow/cpp/build$ git diff
diff --git a/cpp/src/arrow/util/ree_util.h b/cpp/src/arrow/util/ree_util.h
index 5a24024..c6bb4e5 100644
--- a/cpp/src/arrow/util/ree_util.h
+++ b/cpp/src/arrow/util/ree_util.h
@@ -104,6 +104,7 @@ int64_t FindPhysicalLength(const RunEndCType* run_ends, int64_t run_ends_size,
                            int64_t length, int64_t offset) {
   auto [_, physical_length] =
       FindPhysicalRange<RunEndCType>(run_ends, run_ends_size, length, offset);
+  ARROW_UNUSED(_);
   return physical_length;
 }
 
codzilla@hogwarts:~/arrow/cpp/build$ cmake --build .
Recompacting log...
[91/91] Creating library symlink debug/libarrow.so.1300 debug/libarrow.so

[kou]

Thanks!

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 6 benchmarking runs that have been run so far on merge-commit 6c660a5.

There were no benchmark performance regressions. 🎉

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