Add -Wundef to the CI scripts when building with Bazel

The CMake build already adds -Wundef for gcc/clang. This change makes sure that
the gcc/clang Bazel builds also compile correctly with -Wundef (#3267).

PiperOrigin-RevId: 514864451
Change-Id: I7798a4a4c68d037e23625db24ee29df454367734

ci/macos-presubmit.sh

@@ -66,6 +66,7 @@ for absl in 0 1; do
   ${BAZEL_BIN} test ... \
     --copt="-Wall" \
     --copt="-Werror" \
+    --copt="-Wundef" \
     --cxxopt="-std=c++14" \
     --define="absl=${absl}" \
     --features=external_include_paths \