Ignore unknown warning flags when building Abseil

This is intended to provide backward compatibility with older versions
of Clang in Mac host toolchains that do not support -Wno-deprecated-builtins

See https://github.com/flutter/flutter/issues/108388

Change-Id: I0870b57c9ac0bbf294ab93f0d50d928665f2d3b6
Reviewed-on: https://flutter-review.googlesource.com/c/third_party/abseil-cpp/+/32320
Reviewed-by: Chinmay Garde <chinmaygarde@google.com>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index b623246..65d3e56 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -120,8 +120,14 @@
 
 config("absl_include_config") {
   include_dirs = [ "." ]
-  cflags_cc = [ "-Wno-deprecated-builtins" ]
-  cflags_objcc = [ "-Wno-deprecated-builtins" ]
+  cflags_cc = [
+    "-Wno-unknown-warning-option",
+    "-Wno-deprecated-builtins",
+  ]
+  cflags_objcc = [
+    "-Wno-unknown-warning-option",
+    "-Wno-deprecated-builtins",
+  ]
 }
 
 config("absl_define_config") {