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.
 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) {
+  declare_args() {

[brettw, 2014/01/08 00:24:20]

I think this should go in a separate mac SDK .gni file that this one imports.
The reason is that the SDK location doesn't really have anything to do with the
sysroot. If another BUILD.gn file wants the Mac SDK location, it makes more
sense for them to import the SDK .gni file thatn the sysroot one.

You can see my equivalent Windows one:
https://codereview.chromium.org/126073005/

So maybe call it build/config/mac/mac_sdk.gni (or something), move this code
there, and replace it with an import.

[bbudge, 2014/01/08 01:13:15]

Done.

+    # Minimum supported version of the Mac SDK.
+    mac_sdk_min = "10.6"
+
+    # Path to the Mac SDKs, not including a backslash at the end.
+    mac_sdk_path = "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs"
+  }
+
   # 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"
+  if (is_chrome_branded && is_official_build) {
+    find_sdk_args = ["--verify", mac_sdk_min "--sdk_path=" + mac_sdk_path]
+  } else {
+    find_sdk_args = [mac_sdk_min]
+  }
+  mac_sdk_version =
+      exec_script("//build/mac/find_sdk.py", find_sdk_args, "list lines")
+  sysroot = mac_sdk_path + "/MacOSX" + mac_sdk_version[0] + ".sdk"
 } else {
   sysroot = ""
 }