Fix gcc_version.gni to determine target GCC toolchain version for Chrome OS.

build/config/gcc/gcc_version.gni

 # Copyright 2014 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.
 
 if (is_clang) {
   gcc_version = 0
 } else if (is_android) {
   import("//build/config/android/config.gni")
   if (is_android_webview_build) {
     gcc_version = 48
   } else {
     gcc_version = 49
   }
 } else {
-  gcc_version = exec_script("../../compiler_version.py", [ "host",  "compiler" ], "value")
+  gcc_version = exec_script("../../compiler_version.py", [ "target", "compiler" ], "value")

[brettw, 2014/10/29 00:35:22]

Sorry for the delay. This is a bit different than GYP because GYP runs one pass
and gcc_version means target compiler. In GN, this code will run multiple times,
once for each toolchain. This is a bit sticky since it could be any number of
values.

Currently, the toolchain might be nacl, anything defined in
//build/config/linux/BUILD.gn, or //build/toolchain/cros:target

We should define this value for each of those options, defaulting to 0 for cases
we don't handle. The cros:target is clearly the "target" GCC version.

So I'd rewrite this whole condition, removing the is_clang check at the top:

if (is_android) {
  ... current code ...
} else if (current_toolchain == "//build/toolchain/cros:target") {
  gcc_version = exec_script("../../compiler_version.py", [ "target", "compiler"
], "value")
} else if (current_toolchain == "//build/toolchain/linux:x64" ||
           current_toolchain == "//build/toolchain/linux:x86) {
  # These are both the same and just use the default gcc on the system.
  gcc_version = exec_script(.... [ "host", ...)
} else {
  gcc_version = 0
}

 }