Force rebuild of everything if the version of Xcode changes.

build/config/mac/BUILD.gn

 # 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.
 
 import("//build/config/sysroot.gni")
-if (is_ios) {
-  import("//build/config/ios/ios_sdk.gni")
-}
 import("//build/config/mac/mac_sdk.gni")
 import("//build/config/mac/symbols.gni")
 
+if (is_ios) {
+  # This needs to be imported after mac_sdk.gni as it overrides some of the
+  # variables defined by it.

[Dirk Pranke, 2016/08/09 19:55:48]

Told ya! :).

We should modify ios_sdk.gni to either include mac_sdk.gni, or split the common
stuff out into an xcode.gni or something. Right now, it's not obvious that both
files are modifying a shared variable.

+  import("//build/config/ios/ios_sdk.gni")
+}
+
 # This is included by reference in the //build/config/compiler config that
 # is applied to all targets. It is here to separate out the logic.
 #
 # This is applied to BOTH desktop Mac and iOS targets.
 config("compiler") {
   # These flags are shared between the C compiler and linker.
   common_mac_flags = []
 
   # CPU architecture.
   if (current_cpu == "x64") {
@@ skipping 11 matching lines @@
       "-arch",
       "armv7",
     ]
   } else if (current_cpu == "arm64") {
     common_mac_flags += [
       "-arch",
       "arm64",
     ]
   }
 
+  # This is here so that all files get recompiled after an Xcode update.
+  # (defines are passed via the command line, and build system rebuild things
+  # when their commandline changes). Nothing should ever read this define.
+  defines = [ "CR_XCODE_VERSION=$xcode_version" ]
+
   asmflags = common_mac_flags
   cflags = common_mac_flags
 
   # Without this, the constructors and destructors of a C++ object inside
   # an Objective C struct won't be called, which is very bad.
   cflags_objcc = [ "-fobjc-call-cxx-cdtors" ]
 
   cflags_c = [ "-std=c99" ]
   cflags_objc = cflags_c
 
@@ skipping 66 matching lines @@
 # from a binary, but some targets may wish to specify a saves file to preserve
 # specific symbols.
 config("strip_all") {
   # On iOS, the final applications are assembled using lipo (to support fat
   # builds). This configuration is thus always empty and the correct flags
   # are passed to the linker_driver.py script directly during the lipo call.
   if (enable_stripping && !is_ios) {
     ldflags = [ "-Wcrl,strip,-x,-S" ]
   }
 }