Fix for issue #7724: ensure that distutils and python's own setup.py

honor the MacOSX SDK when one is specified.

This is needed to be able to build using the 10.4u SDK while running
on OSX 10.6.

This is a fixed version of the patch in r80963, I've tested this patch
on OSX and Linux.

unixccompiler.py

@@ -15,7 +15,7 @@ the "typical" Unix-style command-line C compiler:
 
 __revision__ = "$Id$"
 
-import os, sys
+import os, sys, re
 from types import StringType, NoneType
 
 from distutils import sysconfig
@@ -305,10 +305,30 @@ class UnixCCompiler(CCompiler):
         dylib_f = self.library_filename(lib, lib_type='dylib')
         static_f = self.library_filename(lib, lib_type='static')
 
+        if sys.platform == 'darwin':
+            # On OSX users can specify an alternate SDK using
+            # '-isysroot', calculate the SDK root if it is specified
+            # (and use it further on)
+            cflags = sysconfig.get_config_var('CFLAGS')
+            m = re.search(r'-isysroot\s+(\S+)', cflags)
+            if m is None:
+                sysroot = '/'
+            else:
+                sysroot = m.group(1)
+
+
+
         for dir in dirs:
             shared = os.path.join(dir, shared_f)
             dylib = os.path.join(dir, dylib_f)
             static = os.path.join(dir, static_f)
+
+            if sys.platform == 'darwin' and (
+                    dir.startswith('/System/') or dir.startswith('/usr/')):
+                shared = os.path.join(sysroot, dir[1:], shared_f)
+                dylib = os.path.join(sysroot, dir[1:], dylib_f)
+                static = os.path.join(sysroot, dir[1:], static_f)
+
             # We're second-guessing the linker here, with not much hard
             # data to go on: GCC seems to prefer the shared library, so I'm
             # assuming that *all* Unix C compilers do.  And of course I'm