build: scope warnings-as-error to project targets, not FetchContent subprojects (#819)

## 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.

Author: yadavay-amzn

CMakeLists.txt

@@ -37,8 +37,6 @@ set(CMAKE_CXX_STANDARD 23)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
-set(CMAKE_COMPILE_WARNING_AS_ERROR ON)
-
 option(ICEBERG_BUILD_STATIC "Build static library" ON)
 option(ICEBERG_BUILD_SHARED "Build shared library" OFF)
 option(ICEBERG_BUILD_TESTS "Build tests" ON)

cmake_modules/IcebergBuildUtils.cmake

@@ -113,6 +113,7 @@ function(add_iceberg_lib LIB_NAME)
 
   if(BUILD_SHARED)
     add_library(${LIB_NAME}_shared SHARED)
+    set_target_properties(${LIB_NAME}_shared PROPERTIES COMPILE_WARNING_AS_ERROR ON)
 
     if(LIB_DEPS)
       target_sources(${LIB_NAME}_shared PRIVATE ${LIB_DEPS})
@@ -172,6 +173,7 @@ function(add_iceberg_lib LIB_NAME)
 
   if(BUILD_STATIC)
     add_library(${LIB_NAME}_static STATIC)
+    set_target_properties(${LIB_NAME}_static PROPERTIES COMPILE_WARNING_AS_ERROR ON)
 
     if(LIB_DEPS)
       target_sources(${LIB_NAME}_static PRIVATE ${LIB_DEPS})

cmake_modules/IcebergThirdpartyToolchain.cmake

@@ -151,6 +151,10 @@ endmacro()
 function(resolve_arrow_dependency)
   prepare_fetchcontent()
 
+  # Prevent Arrow from injecting -Werror into CMAKE_CXX_FLAGS_DEBUG via
+  # arrow_add_werror_if_debug(). PRODUCTION level only adds standard warnings.
+  set(BUILD_WARNING_LEVEL PRODUCTION)
+
   set(ARROW_BUILD_SHARED OFF)
   set(ARROW_BUILD_STATIC ON)
   # Work around undefined symbol: arrow::ipc::ReadSchema(arrow::io::InputStream*, arrow::ipc::DictionaryMemo*)

src/iceberg/test/CMakeLists.txt

@@ -15,8 +15,6 @@
 # specific language governing permissions and limitations
 # under the License.
 
-set(CMAKE_COMPILE_WARNING_AS_ERROR FALSE)
-
 set(INSTALL_GTEST
     OFF
     CACHE BOOL "" FORCE)