Add a snapshot of flutter/engine/src/build to our sdk

build/config/ios/BUILD.gn

Index: build/config/ios/BUILD.gn
diff --git a/build/config/ios/BUILD.gn b/build/config/ios/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..02923155891a23f28a270b262b601bf427e5c971
--- /dev/null
+++ b/build/config/ios/BUILD.gn
@@ -0,0 +1,15 @@
+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/config/sysroot.gni")
+import("//build/config/ios/ios_sdk.gni")
+
+config("sdk") {
+  common_flags = [
+    "-stdlib=libc++",
+  ]
+
+  cflags = common_flags
+  ldflags = common_flags
+}