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

build/config/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.
+# For official linux 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"
+  import("//build/config/mac/mac_sdk.gni")
+
+  sysroot = mac_sdk_path
 } else {
   sysroot = ""
 }