Issue #13590: Improve support for OS X Xcode 4:
- fix test_distutils and test_sysconfig test failures by
  aligning sysconfig and distutils.sysconfig tailoring of
  configure variables (as in 2.7)
diff --git a/Lib/distutils/sysconfig.py b/Lib/distutils/sysconfig.py
index dac3035..f6e5d99 100644
--- a/Lib/distutils/sysconfig.py
+++ b/Lib/distutils/sysconfig.py
@@ -624,7 +624,7 @@
                 # are in CFLAGS or LDFLAGS and remove them if they are.
                 # This is needed when building extensions on a 10.3 system
                 # using a universal build of python.
-                for key in ('LDFLAGS', 'BASECFLAGS',
+                for key in ('LDFLAGS', 'BASECFLAGS', 'LDSHARED',
                         # a number of derived variables. These need to be
                         # patched up as well.
                         'CFLAGS', 'PY_CFLAGS', 'BLDSHARED'):
@@ -669,16 +669,39 @@
                 # that OS release.
                 if 'ARCHFLAGS' in os.environ:
                     arch = os.environ['ARCHFLAGS']
-                    for key in ('LDFLAGS', 'BASECFLAGS',
+                    for key in ('LDFLAGS', 'BASECFLAGS', 'LDSHARED',
                         # a number of derived variables. These need to be
                         # patched up as well.
-                        'CFLAGS', 'PY_CFLAGS', 'BLDSHARED', 'LDSHARED'):
+                        'CFLAGS', 'PY_CFLAGS', 'BLDSHARED'):
 
                         flags = _config_vars[key]
                         flags = re.sub('-arch\s+\w+\s', ' ', flags)
                         flags = flags + ' ' + arch
                         _config_vars[key] = flags
 
+                # If we're on OSX 10.5 or later and the user tries to
+                # compiles an extension using an SDK that is not present
+                # on the current machine it is better to not use an SDK
+                # than to fail.
+                #
+                # The major usecase for this is users using a Python.org
+                # binary installer  on OSX 10.6: that installer uses
+                # the 10.4u SDK, but that SDK is not installed by default
+                # when you install Xcode.
+                #
+                m = re.search('-isysroot\s+(\S+)', _config_vars['CFLAGS'])
+                if m is not None:
+                    sdk = m.group(1)
+                    if not os.path.exists(sdk):
+                        for key in ('LDFLAGS', 'BASECFLAGS', 'LDSHARED',
+                             # a number of derived variables. These need to be
+                             # patched up as well.
+                            'CFLAGS', 'PY_CFLAGS', 'BLDSHARED'):
+
+                            flags = _config_vars[key]
+                            flags = re.sub('-isysroot\s+\S+(\s|$)', ' ', flags)
+                            _config_vars[key] = flags
+
     if args:
         vals = []
         for name in args: