Support DEVELOPER_DIR in gyp-ninja for mac.

pylib/gyp/xcode_emulation.py

Index: pylib/gyp/xcode_emulation.py
diff --git a/pylib/gyp/xcode_emulation.py b/pylib/gyp/xcode_emulation.py
index 73a7f69b452d2b340849647755e8f7cec27a7325..2dec19e5d6fb548bd2002c37f88dbcf0086eecab 100644
--- a/pylib/gyp/xcode_emulation.py
+++ b/pylib/gyp/xcode_emulation.py
@@ -162,6 +162,7 @@ class XcodeSettings(object):
     self.spec = spec
 
     self.isIOS = False
+    self.mac_toolchain_dir = None
     self.header_map_path = None
 
     # Per-target 'xcode_settings' are pushed down into configs earlier by gyp.
@@ -1515,6 +1516,9 @@ def _GetXcodeEnv(xcode_settings, built_products_dir, srcroot, configuration,
   else:
     env['SDKROOT'] = ''
 
+  if xcode_settings.mac_toolchain_dir:
+    env['DEVELOPER_DIR'] = xcode_settings.mac_toolchain_dir
+
   if spec['type'] in (
       'executable', 'static_library', 'shared_library', 'loadable_module'):
     env['EXECUTABLE_NAME'] = xcode_settings.GetExecutableName()