Fix -Wundef compilation

This allows downstream consumers of protobuf to enable
Wundef in their code which consumes protobuf APIs.

This change also enables warnings in the ci bazelrc to ensure that
no new warning failures are introduced.

ci/common.bazelrc

@@ -33,6 +33,17 @@ build:ubsan --copt=-fno-sanitize=function --copt=-fno-sanitize=vptr
 # Abseil passes nullptr to memcmp with 0 size
 build:ubsan --copt=-fno-sanitize=nonnull-attribute
 
+# Check for common warnings to allow for strict downstream warning checks.
+build --copt=-Wundef
+build --copt=-Wpointer-arith
+# Build currently fails under:
+# build --copt=-Wshadow
+# build --copt=-Wredundant-decls
+# build --copt=-Wall
+# build --copt=-Wextra
+# build --copt=-Wimplicit-fallthrough
+# build --copt=-Wcast-qual
+
 # Workaround Bazel 7 remote cache issues.
 # See https://github.com/bazelbuild/bazel/issues/20161
 build --experimental_remote_cache_eviction_retries=5

toolchain/cc_toolchain_config.bzl

@@ -146,6 +146,7 @@ def _impl(ctx):
                         flags = [
                             bit_flag,
                             "-Wall",
+                            "-Wundef",
                             "-no-canonical-prefixes",
                             "--target=" + ctx.attr.target_full_name,
                             "-fvisibility=hidden",