diff --git a/CMakeLists.txt b/CMakeLists.txt
index c6a1482f..93654055 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -14,7 +14,6 @@ project(s2-geometry
         VERSION 0.12.0)
 
 include(CMakeDependentOption)
-include(CheckCXXCompilerFlag)
 include(FeatureSummary)
 include(FetchContent)
 include(FindPackageHandleStandardArgs)
@@ -123,19 +122,21 @@ else()
     # util/math/vector.h:178:16: warning: optimization attribute on
     # ‘double sqrt(double)’ follows definition but the attribute doesn’t
     # match [-Wattributes]
-    add_definitions(-Wno-attributes)
-    add_definitions(-Wno-deprecated-declarations)
-    add_definitions(-Wno-nullability-completeness)
+    add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-Wno-attributes>)
+    add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-Wno-deprecated-declarations>)
+    add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-Wno-sign-compare>)
+    add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-Wno-nullability-completeness>)
     # Suppress noisy AArch64 ABI notes about parameter passing changes in
     # GCC 10.1.  Irrelevant since we don't mix objects from old compilers.
-    # TODO: Use check_cxx_compiler_flag(-Wno-psabi) to guard this.
-    add_compile_options(-Wno-psabi)
+    add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-Wno-psabi>)
     # Some files use sized deallocation, which should be enabled by
     # default for C++14 and later.  There appears to be a bug with clang
     # < 19 causing this not to be enabled.
     # https://github.com/google/s2geometry/issues/411#issuecomment-2726949607
     # This can be removed when clang19 is the minimum supported version.
-    add_compile_options(-fsized-deallocation)
+    if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+        add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-fsized-deallocation>)
+    endif()
 endif()
 
 # If OpenSSL is installed in a non-standard location, configure with
diff --git a/src/.bazelrc b/src/.bazelrc
index 145802fd..e79fd655 100644
--- a/src/.bazelrc
+++ b/src/.bazelrc
@@ -3,10 +3,28 @@ common --enable_bzlmod
 common --cxxopt=-std=c++20
 common --announce_rc
 
+# C++ warning flags for non-MSVC toolchains. Keep in line with CMakeLists.txt.
+# `bazel test --config=ci` uses `build:ci --config=gcc` (Linux CI). For local
+# macOS / Clang builds, pass `--config=clang` in addition to your other flags.
+
+# Host GCC (Linux CI).
+build:gcc --cxxopt=-Wno-attributes
+build:gcc --cxxopt=-Wno-deprecated-declarations
+build:gcc --cxxopt=-Wno-sign-compare
+build:gcc --cxxopt=-Wno-psabi
+
+# Clang (e.g. macOS).
+build:clang --cxxopt=-Wno-attributes
+build:clang --cxxopt=-Wno-deprecated-declarations
+build:clang --cxxopt=-Wno-sign-compare
+build:clang --cxxopt=-Wno-nullability-completeness
+build:clang --cxxopt=-fsized-deallocation
+
 # Faster tests than fastbuild (-O0).
 build:dev --copt=-O1
 
-# CI-specific flags
+# CI-specific flags.
 common:ci --color=yes
+build:ci --config=gcc
 build:ci --show_progress_rate_limit=10
 test:ci --test_output=errors