bazel: Silence Protobuf and zlib compilation warnings

Author: fmeum

.bazelrc

@@ -12,6 +12,9 @@ build --nozip_undeclared_test_outputs
 common --repo_env=CC=clang
 # Required by abseil-cpp.
 build --cxxopt=-std=c++14
+# Silence protobuf compilation warnings.
+build --host_copt=-Wno-unused-function
+build --host_copt=-Wno-unknown-warning-option
 # Requires a relatively modern clang.
 build:ci --features=layering_check
 build:macos --apple_crosstool_top=@local_config_apple_cc//:toolchain