bpo-41116: Ensure system supplied libraries are found on macOS 11 (GH-23301)

On macOS system provided libraries are in a shared library cache
and not at their usual location. This PR teaches distutils to search
in the SDK, even if there was no "-sysroot" argument in
the compiler flags.

setup.py

@@ -9,6 +9,7 @@ import re
 import sys
 import sysconfig
 from glob import glob, escape
+import _osx_support
 
 
 try:
@@ -176,34 +177,10 @@ def macosx_sdk_root():
     m = re.search(r'-isysroot\s*(\S+)', cflags)
     if m is not None:
         MACOS_SDK_ROOT = m.group(1)
-        MACOS_SDK_SPECIFIED = MACOS_SDK_ROOT != '/'
     else:
-        MACOS_SDK_ROOT = '/'
-        MACOS_SDK_SPECIFIED = False
-        cc = sysconfig.get_config_var('CC')
-        tmpfile = '/tmp/setup_sdk_root.%d' % os.getpid()
-        try:
-            os.unlink(tmpfile)
-        except:
-            pass
-        ret = run_command('%s -E -v - </dev/null 2>%s 1>/dev/null' % (cc, tmpfile))
-        in_incdirs = False
-        try:
-            if ret == 0:
-                with open(tmpfile) as fp:
-                    for line in fp.readlines():
-                        if line.startswith("#include <...>"):
-                            in_incdirs = True
-                        elif line.startswith("End of search list"):
-                            in_incdirs = False
-                        elif in_incdirs:
-                            line = line.strip()
-                            if line == '/usr/include':
-                                MACOS_SDK_ROOT = '/'
-                            elif line.endswith(".sdk/usr/include"):
-                                MACOS_SDK_ROOT = line[:-12]
-        finally:
-            os.unlink(tmpfile)
+        MACOS_SDK_ROOT = _osx_support._default_sysroot(
+            sysconfig.get_config_var('CC'))
+    MACOS_SDK_SPECIFIED = MACOS_SDK_ROOT != '/'
 
     return MACOS_SDK_ROOT