Allow use of ^blocks, even with the 10.5 SDK, and even with a 10.5 runtime

chrome/chrome_browser.gypi

Index: chrome/chrome_browser.gypi
===================================================================
--- chrome/chrome_browser.gypi	(revision 95801)
+++ chrome/chrome_browser.gypi	(working copy)
@@ -4905,4 +4905,55 @@
       'hard_dependency': 1,
     },
   ],
+  'conditions': [
+    ['OS=="mac"', {
+      'targets': [
+        {
+          'target_name': 'closure_blocks_leopard_compat',
+          'conditions': [
+            ['mac_sdk == "10.5"', {
+              'type': 'shared_library',
+              'product_name': 'closure_blocks_leopard_compat_stub',
+              'sources': [
+                'browser/mac/closure_blocks_leopard_compat.S',
+              ],
+              'xcode_settings': {
+                # These values are taken from libSystem.dylib in the 10.5 SDK.
+                # Setting LD_DYLIB_INSTALL_NAME causes anything linked against
+                # this stub library to look for the symbols it provides in the
+                # real libSystem at runtime. The real library's compatibility
+                # version is used, and the value of the current version from
+                # the SDK is used to make it appear as though anything linked 
+                # against this stub was linked against the real thing.
+                'LD_DYLIB_INSTALL_NAME': '/usr/lib/libSystem.B.dylib',
+                'DYLIB_COMPATIBILITY_VERSION': '1.0.0',
+                'DYLIB_CURRENT_VERSION': '111.1.4',
+
+                # Turn on stripping (yes, even in debug mode), and add the -c
+                # flag. This is what produces a stub library (MH_DYLIB_STUB)
+                # as opposed to a dylib (MH_DYLIB). MH_DYLIB_STUB files
+                # contain symbol tables and everything else needed for
+                # linking, but are stripped of section contents. This is the
+                # same way that the stub libraries in Mac OS X SDKs are
+                # created. dyld will refuse to load a stub library, so this
+                # provides some insurance in case anyone tries to load the
+                # stub at runtime.
+                'DEPLOYMENT_POSTPROCESSING': 'YES',
+                'STRIP_STYLE': 'non-global',
+                'STRIPFLAGS': '-c',
+              },
+            }, {  # else: mac_sdk != "10.5"
+              # When using the 10.6 SDK or newer, the necessary definitions
+              # are already present in libSystem.dylib. There is no need to
+              # build a stub dylib to provide these symbols at link time. This
+              # target is still useful to cause those symbols to be treated as
+              # weak imports in dependents, who still must #include
+              # closure_blocks_leopard_compat.h to get weak imports.
+              'type': 'none',
+            }],
+          ],
+        },
+      ],
+    }],
+  ],
 }