build: scope warnings-as-error to project targets, not FetchContent subprojects

[yadavay-amzn]

What

Fixes #246. -Werror (via a global CMAKE_COMPILE_WARNING_AS_ERROR) leaked into FetchContent-built dependencies, so a warning in a third-party subproject (e.g. Arrow) could fail the whole build. As noted on the issue, the global flag does not reliably isolate subprojects.

How

• Removed the global CMAKE_COMPILE_WARNING_AS_ERROR ON.
• Set COMPILE_WARNING_AS_ERROR ON as a per-target property on Iceberg's own shared/static libraries, so the project stays strict.
• Set BUILD_WARNING_LEVEL PRODUCTION for Arrow to suppress its independent -Werror injection.
• Kept a defensive guard in prepare_fetchcontent() for the case where -DCMAKE_COMPILE_WARNING_AS_ERROR=ON is passed on the command line.

Verified

Iceberg targets still compile with -Werror; FetchContent subprojects do not. Build + tests green (18/18).

---

This contribution was authored with assistance from Claude Opus 4.8, in line with the Iceberg guidelines for AI-assisted contributions (https://iceberg.apache.org/contribute/#guidelines-for-ai-assisted-contributions). All changes were reviewed and tested by the author.

[zhjwpku]

Could you please configure pre-commit locally? It seems the patch contains some unnecessary changes.

[yadavay-amzn]

Done - configured pre-commit locally with the pinned cmake-format (v0.6.10), reverted the unrelated cosmetic reformattings, and re-pushed. The diff now contains only the warnings-as-error scoping change and passes pre-commit.

[evindj]

The change looks good to me. However, I wonder if this comes from an hypothetical or an actual occurence where the dependencies triggering the issue was not able/willing to address. I generally agree that we should do this but I wonder if relaxing the requirement causes side effects.