Change the canonical way to include the C macros.h to <mojo/macros.h>.

mojo/public/c/system/tests/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("../../../mojo_sdk.gni")
 
 mojo_sdk_source_set("tests") {
   testonly = true
 
   cflags_c = [ "-Wundef" ]
@@ skipping 10 matching lines @@
     "wait_set_unittest.cc",
     "wait_unittest.cc",
   ]
 
   deps = [
     ":pure_tests",
     "//testing/gtest",
   ]
 
   mojo_sdk_deps = [
+    "mojo/public/c:system",
     "mojo/public/c/environment",
     "mojo/public/c/system",
   ]
 }
 
 # This source set contains "pure" tests that only depend on the public SDK (and
 # the standard library), and in particular not on gtest (whose headers don't
 # compile with "-Wundef").
 mojo_sdk_source_set("pure_tests") {
   testonly = true
 
   visibility = [ ":tests" ]
 
   cflags = [ "-Wundef" ]
 
   sources = [
     "compile_unittest_pure_c.c",
     "compile_unittest_pure_cpp.cc",
   ]
 
   mojo_sdk_deps = [
+    "mojo/public/c:system",
     "mojo/public/c/environment",
     "mojo/public/c/system",
   ]
 }
 
 mojo_sdk_source_set("perftests") {
   testonly = true
 
   sources = [
     "message_pipe_perftest.cc",
     "perftest_utils.cc",
     "perftest_utils.h",
     "reference_perftest.cc",
     "wait_set_perftest.cc",
   ]
 
   deps = [
     "//testing/gtest",
   ]
 
   mojo_sdk_deps = [
+    "mojo/public/c:system",
     "mojo/public/c/environment",
     "mojo/public/c/system",
     "mojo/public/cpp/test_support",
   ]
 }