Revert of Mojo: Eliminate duplicate C API symbols

mojo/edk/system/BUILD.gn

 # 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/nacl/config.gni")
 import("//testing/test.gni")
 import("../../../mojo/public/tools/bindings/mojom.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
   import("//build/config/android/rules.gni")
 }
 
+config("system_config") {
+  defines = [
+    # Ensures that dependent projects import the core functions on Windows.
+    "MOJO_USE_SYSTEM_IMPL",
+  ]
+}
+
 component("system") {
   output_name = "mojo_system_impl"
 
   sources = [
     "async_waiter.cc",
     "async_waiter.h",
     "atomic_flag.h",
     "awakable.h",
     "awakable_list.cc",
     "awakable_list.h",
@@ skipping 44 matching lines @@
     "wait_set_dispatcher.cc",
     "wait_set_dispatcher.h",
     "waiter.cc",
     "waiter.h",
     "watcher.cc",
     "watcher.h",
     "watcher_set.cc",
     "watcher_set.h",
   ]
 
-  defines = [ "MOJO_SYSTEM_IMPL_IMPLEMENTATION" ]
+  defines = [
+    "MOJO_SYSTEM_IMPL_IMPLEMENTATION",
+    "MOJO_SYSTEM_IMPLEMENTATION",
+  ]
+
+  all_dependent_configs = [ ":system_config" ]
 
   public_deps = [
     "//mojo/edk/embedder",
     "//mojo/edk/embedder:delegates",
     "//mojo/edk/embedder:platform",
     "//mojo/edk/system/ports",
     "//mojo/public/c/system",
     "//mojo/public/cpp/system",
   ]
 
@@ skipping 112 matching lines @@
       ":test_utils",
       "//base",
       "//base/test:test_support",
       "//mojo/edk/system",
       "//mojo/edk/test:run_all_perftests",
       "//mojo/edk/test:test_support",
       "//testing/gtest",
     ]
   }
 }