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

build/common.gypi

 # Copyright (c) 2012 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 624 matching lines @@
     # "Maximum allowed" refers to the operating system version whose APIs are
     # available in the headers.
     # The deployment target identifies the minimum system version that the
     # built products are expected to function on.  It corresponds to the
     # MAC_OS_X_VERSION_MIN_REQUIRED compile-time macro.
     # To ensure these macros are available, #include <AvailabilityMacros.h>.
     # Additional documentation on these macros is available at
     # http://developer.apple.com/mac/library/technotes/tn2002/tn2064.html#SECTION3
     # Chrome normally builds with the Mac OS X 10.6 SDK and sets the
     # deployment target to 10.5.  Other projects, such as O3D, may override
-    # these defaults.
+    # these defaults. If the SDK is installed someplace that Xcode doesn't
+    # know about, set mac_sdk_path to the path to the SDK. If set to a
+    # non-empty string, mac_sdk_path will be used in preference to mac_sdk.
     'mac_sdk%': '10.6',
+    'mac_sdk_path%': '',
     'mac_deployment_target%': '10.5',
 
     # The default value for mac_strip in target_defaults. This cannot be
     # set there, per the comment about variable% in a target_defaults.
     'mac_strip_release%': 1,
 
     # Set to 1 to enable code coverage.  In addition to build changes
     # (e.g. extra CFLAGS), also creates a new target in the src/chrome
     # project file called "coverage".
     # Currently ignored on Windows.
@@ skipping 2632 matching lines @@
       ],
     }],
   ],
   'xcode_settings': {
     # DON'T ADD ANYTHING NEW TO THIS BLOCK UNLESS YOU REALLY REALLY NEED IT!
     # This block adds *project-wide* configuration settings to each project
     # file.  It's almost always wrong to put things here.  Specify your
     # custom xcode_settings in target_defaults to add them to targets instead.
 
     'conditions': [
-      ['OS=="mac"', {
-        # In an Xcode Project Info window, the "Base SDK for All Configurations"
-        # setting sets the SDK on a project-wide basis.  In order to get the
-        # configured SDK to show properly in the Xcode UI, SDKROOT must be set
-        # here at the project level.
-        'SDKROOT': 'macosx<(mac_sdk)',  # -isysroot
+      # In an Xcode Project Info window, the "Base SDK for All Configurations"
+      # setting sets the SDK on a project-wide basis. In order to get the
+      # configured SDK to show properly in the Xcode UI, SDKROOT must be set
+      # here at the project level.
+      ['mac_sdk_path==""', {
+        'conditions': [
+          ['OS=="mac"', {
+            'SDKROOT': 'macosx<(mac_sdk)',  # -isysroot
+          }],
+          ['OS=="ios"', {
+            'SDKROOT': 'iphoneos<(ios_sdk)',  # -isysroot
+          }],
+        ],
+      }, {  # else: mac_sdk_path!=""
+        'SDKROOT': '<(mac_sdk_path)',  # -isysroot
       }],
       ['OS=="ios"', {
         # Just build armv7 since iOS 4.3+ only supports armv7.
         'ARCHS': '$(ARCHS_UNIVERSAL_IPHONE_OS)',
         'IPHONEOS_DEPLOYMENT_TARGET': '<(ios_deployment_target)',
-        'SDKROOT': 'iphoneos<(ios_sdk)',  # -isysroot
         # Target both iPhone and iPad.
         'TARGETED_DEVICE_FAMILY': '1,2',
       }],
     ],
 
     # The Xcode generator will look for an xcode_settings section at the root
     # of each dict and use it to apply settings on a file-wide basis.  Most
     # settings should not be here, they should be in target-specific
     # xcode_settings sections, or better yet, should use non-Xcode-specific
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # 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',
   },
 }