Silence C++ compilation warnings from external dependencies in the Bazel config.

PiperOrigin-RevId: 646978405
Change-Id: I5e68a04897660c7537b9d7302895adcd9250722e

Author: chuckx

.bazelrc

@@ -1,8 +1,16 @@
 # TODO: b/332650707 - Explicitly disable bzlmod until supported.
 always --noenable_bzlmod
+
 # Minumum C++ version. Override it building this project with
 # `bazel build --cxxopt='-std=c++<XY>' --host_cxxopt='c++<XY>' ...`
 # (Both -std and --host_cxxopt must be set to force the desired version.)
 build --cxxopt='-std=c++17' --host_cxxopt='-std=c++17'
 build --java_language_version=8
 build --java_runtime_version=local_jdk
+
+# Silence all C/C++ warnings in external code.
+#
+# Note that this will not silence warnings from external headers included
+# in project code.
+build --per_file_copt=external/.*@-w
+build --host_per_file_copt=external/.*@-w