code: def _check_for_unavailable_sdk(_config_vars):
      """Remove references to any SDKs not available"""
      # If we're on OSX 10.5 or later and the user tries to
      # compile an extension using an SDK that is not present
      # on the current machine it is better to not use an SDK
      # than to fail.  This is particularly important with
      # the standalone Command Line Tools alternative to a
      # full-blown Xcode install since the CLT packages do not
      # provide SDKs.  If the SDK is not present, it is assumed
      # that the header files and dev libs have been installed
      # to /usr and /System/Library by either a standalone CLT
      # package or the CLT component within Xcode.
      cflags = _config_vars.get("CFLAGS", "")
      m = re.search(r"-isysroot\s*(\S+)", cflags)
      if m is not None:
          sdk = m.group(1)
          if not os.path.exists(sdk):
              for cv in _UNIVERSAL_CONFIG_VARS:
                  # Do not alter a config var explicitly overridden by env var
                  if cv in _config_vars and cv not in os.environ:
                      flags = _config_vars[cv]
                      flags = re.sub(
                          r"-isysroot\s*\S+(?:\s|$)", " ", flags
                      )
                      _save_modified_value(_config_vars, cv, flags)
  
      return _config_vars
  
imports: import os
  import re
  import sys
label_desc: Use image node_id 15 for calling a global function with example usage: _check_for_unavailable_sdk(_config_vars) and returns: _config_vars