Avoid undefined variable in gyp files.

tools/gyp/xcode.gypi

 # Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 {
   'variables': {
     # Mac OS X SDK and deployment target support.
     # The SDK identifies the version of the system headers that will be used,
     # and corresponds to the MAC_OS_X_VERSION_MAX_ALLOWED compile-time macro.
     # "Maximum allowed" refers to the operating system version whose APIs are
@@ skipping 34 matching lines @@
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
 
     # It is here to make it possible to run binaries with shared libraries.
     # However, it may create problems if you move the shared library. Also,
     # this may not be appropriate for "release release" builds.
     # Perhaps we should set it to @rpath instead. See
     # http://developer.apple.com/library/mac/#documentation/DeveloperTools/Conceptual/DynamicLibraries/100-Articles/RunpathDependentLibraries.html#//apple_ref/doc/uid/TP40008306-SW1
     'INSTALL_PATH': '$(TARGET_BUILD_DIR)',
+
+    # This must go here because mac_deployment_target is defined in this file,
+    # which is only included on mac, and configurations_xcode.gypi is included
+    # on all platforms, so it would be an undefined variable there.
+    # Move this to configurations_xcode if we start including it only on mac.
+    'MACOSX_DEPLOYMENT_TARGET': '<(mac_deployment_target)',
   },
 }