bpo-41116: Ensure system supplied libraries are found on macOS 11 (GH-23301) (GH-23455)

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.
(cherry picked from commit 404a719b51)

Co-authored-by: Ronald Oussoren <ronaldoussoren@mac.com>

Lib/distutils/unixccompiler.py

@@ -290,7 +290,7 @@ def find_library_file(self, dirs, lib, debug=0):
             cflags = sysconfig.get_config_var('CFLAGS')
             m = re.search(r'-isysroot\s*(\S+)', cflags)
             if m is None:
-                sysroot = '/'
+                sysroot = _osx_support._default_sysroot(sysconfig.get_config_var('CC'))
             else:
                 sysroot = m.group(1)