Force rebuild of everything if the version of Xcode changes.

build/config/mac/BUILD.gn

Index: build/config/mac/BUILD.gn
diff --git a/build/config/mac/BUILD.gn b/build/config/mac/BUILD.gn
index ec66a15ce5ec15ca9bcf66f5579d994ce5862dcf..84180e6a65af14b4c9f887498bb8463365b4ed41 100644
--- a/build/config/mac/BUILD.gn
+++ b/build/config/mac/BUILD.gn
@@ -3,11 +3,14 @@
 # found in the LICENSE file.
 
 import("//build/config/sysroot.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")
 }
-import("//build/config/mac/mac_sdk.gni")
-import("//build/config/mac/symbols.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.
@@ -40,6 +43,11 @@ config("compiler") {
     ]
   }
 
+  # 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