Derive default for ar_host from ar.

test/win/gyptest-crosscompile-ar.py

+#!/usr/bin/env python
+
+# Copyright (c) 2015 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.
+
+"""
+Verifies that ar_host is set correctly when enabling cross-compile on windows.
+"""
+
+import TestGyp
+
+import sys
+import os
+
+if sys.platform == 'win32':
+  test = TestGyp.TestGyp(formats=['ninja'])
+
+  CHDIR = 'lib-crosscompile'
+  oldenv = os.environ.copy()
+  try:
+    os.environ['GYP_CROSSCOMPILE'] = '1'
+    test.run_gyp('use_host_ar.gyp', chdir=CHDIR)
+  finally:
+    os.environ.clear()
+    os.environ.update(oldenv)
+
+  test.build('use_host_ar.gyp', test.ALL, chdir=CHDIR)
+  test.pass_test()