GN: Get Mac build working for IPC target at least.

build/config/linux/sysroot.gni

-# Copyright (c) 2013 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.
-
-# This header file defines the "sysroot" variable which is the absolute path
-# of the sysroot. If no sysroot applies, the variable will be an empty string.
-
-# For official builds, use the sysroot checked into the internal source repo so
-# that the builds work on older versions of Linux.
-if (is_linux && is_chrome_branded && is_official_build && !is_chromeos) {
-  if (cpu_arch == "x64") {
-    sysroot = rebase_path(
-      "//chrome/installer/linux/debian_wheezy_amd64-sysroot", ".", "")
-  } else if (cpu_arch == "x86") {
-    sysroot = rebase_path(
-      "//chrome/installer/linux/debian_wheezy_i386-sysroot", ".", "")
-  } else {
-    # Any other builds don't use a sysroot.
-    sysroot = ""
-  }
-} else if (is_mac) {
-  # Set which SDK to use.
-  # TODO(brettw) this needs to be configurable somehow.
-  sysroot = "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.7.sdk"
-} else {
-  sysroot = ""
-}