Merge pull request #3919 from jart/less-warnings

Remove unhelpful build warnings

BUILD

@@ -24,6 +24,8 @@ COPTS = select({
         "-Woverloaded-virtual",
         "-Wno-sign-compare",
         "-Wno-unused-function",
+        # Prevents ISO C++ const string assignment warnings for pyext sources.
+        "-Wno-writable-strings",
     ],
 })