build: avoid duplicate libraries in the C++20 compatibility test (#1015)

Author: wgtmac

src/iceberg/test/CMakeLists.txt

@@ -377,6 +377,16 @@ if(ICEBERG_BUILD_HIVE)
 endif()
 
 if(ICEBERG_CXX20_COMPAT)
+  # De-duplicate libraries only when the linker supports it, without changing
+  # policies for other targets or requiring a newer CMake version.
+  cmake_policy(PUSH)
+  if(POLICY CMP0156)
+    cmake_policy(SET CMP0156 NEW)
+  endif()
+  if(POLICY CMP0179)
+    cmake_policy(SET CMP0179 NEW)
+  endif()
+
   # Compile the public headers as a C++20 consumer. The list lives in
   # iceberg/cpp20_compatibility_internal.h, generated by
   # ci/scripts/generate_public_headers.py; headers of optional components are enabled
@@ -428,4 +438,5 @@ if(ICEBERG_CXX20_COMPAT)
     target_compile_options(cpp20_compatibility_test PRIVATE /bigobj /Zc:preprocessor)
   endif()
   add_test(NAME cpp20_compatibility_test COMMAND cpp20_compatibility_test)
+  cmake_policy(POP)
 endif()