mac: Let mac_sdk default to the oldest available SDK that's at least 10.6

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index c55f65e8d36a63962197b96e2789ccb17f57aaba..bc780f62148f6aac7ffbf8ad32835b59c94739e8 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -638,7 +638,6 @@
     # 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.

[Mark Mentovai, 2012/07/27 19:01:54]

Now that mac_sdk is not set here where this whole descriptive comment talking
about SDKs is, and where mac_sdk_path is, you should probably say that mac_sdk
is normally used to control this, but that it’s set somewhere else.

-    'mac_sdk%': '10.6',
     'mac_sdk_path%': '',
     'mac_deployment_target%': '10.5',
 
@@ -1019,6 +1018,7 @@
           }],
 
           ['branding=="Chrome" and buildtype=="Official"', {
+            'mac_sdk%': '10.6',

[Mark Mentovai, 2012/07/27 19:01:54]

Since you’re doing the condition thing anyway, and you’re inside OS=="mac", you
can actually make the script raise an error on non-Mac.

             # Enable uploading crash dumps.
             'mac_breakpad_uploads%': 1,
             # Enable dumping symbols at build time for use by Mac Breakpad.
@@ -1026,6 +1026,7 @@
             # Enable Keystone auto-update support.
             'mac_keystone%': 1,
           }, { # else: branding!="Chrome" or buildtype!="Official"
+            'mac_sdk%': '<!(python mac/find_sdk.py)',

[Mark Mentovai, 2012/07/27 19:01:54]

I’m slightly inclined to pass 10.6 as an argument to the script from here so
this file is a one-stop shop for setting the [minimum] SDK version, but I don’t
feel strongly enough about it if you don’t like that idea.

             'mac_breakpad_uploads%': 0,
             'mac_breakpad%': 0,
             'mac_keystone%': 0,