code: def _get_flag_value_from_var(flag, var, delim=' '):
      
  
      if sys.platform.startswith('win'):
          return None
  
      # Simple input validation
      if not var or not flag:
          return None
      flag_length = len(flag)
      if not flag_length:
          return None
  
      # Look for var in os.eviron then in get_config_var
      if var in os.environ:
          flags = os.environ[var]
      else:
          try:
              flags = get_config_var(var)
          except KeyError:
              return None
  
      # Extract flag from {var:value}
      if flags:
          for item in flags.split(delim):
              if item.startswith(flag):
                  return item[flag_length:]
docs: Extract flags from an environment variable.
  
      Parameters
      ----------
      flag : str
          The flag to extract, for example '-I' or '-L'
      var : str
          The environment variable to extract the flag from, e.g. CFLAGS or LDFLAGS.
      delim : str, optional
          The delimiter separating flags inside the environment variable
  
      Examples
      --------
      Let's assume the LDFLAGS is set to '-L/usr/local/include -customflag'. This
      function will then return the following:
  
          >>> _get_flag_value_from_var('-L', 'LDFLAGS')
          '/usr/local/include'
  
      Notes
      -----
      Environment variables are first checked in ``os.environ[var]``, then in
      ``distutils.sysconfig.get_config_var(var)``.
  
      This function is not supported on Windows.