Allow the mac_sdk_path GYP variable to set the Mac OS X SDK by path

DEPS

 # When adding a new dependency, please update the top-level .gitignore file
 # to list the dependency's destination directory.
 
 vars = {
   # Use this googlecode_url variable only if there is an internal mirror for it.
   # If you do not know, use the full path while defining your new deps entry.
   "googlecode_url": "http://%s.googlecode.com/svn",
   "sourceforge_url": "http://%(repo)s.svn.sourceforge.net/svnroot/%(repo)s",
   "webkit_trunk": "http://svn.webkit.org/repository/webkit/trunk",
   "nacl_trunk": "http://src.chromium.org/native_client/trunk",
@@ skipping 110 matching lines @@
   "src/third_party/leveldatabase/src":
     (Var("googlecode_url") % "leveldb") + "/trunk@67",
 
   "src/third_party/snappy/src":
     (Var("googlecode_url") % "snappy") + "/trunk@63",
 
   "src/tools/grit":
     (Var("googlecode_url") % "grit-i18n") + "/trunk@63",
 
   "src/tools/gyp":
-    (Var("googlecode_url") % "gyp") + "/trunk@1439",
+    (Var("googlecode_url") % "gyp") + "/trunk@1440",
 
   "src/v8":
     (Var("googlecode_url") % "v8") + "/trunk@" + Var("v8_revision"),
 
   "src/native_client":
     Var("nacl_trunk") + "/src/native_client@" + Var("nacl_revision"),
 
   "src/native_client_sdk/src/site_scons":
     Var("nacl_trunk") + "/src/native_client/site_scons@" + Var("nacl_revision"),
 
@@ skipping 422 matching lines @@
     "pattern": ".",
     "action": ["python", "src/build/util/lastchange.py",
                "-o", "src/build/util/LASTCHANGE"],
   },
   {
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     "pattern": ".",
     "action": ["python", "src/build/gyp_chromium"],
   },
 ]