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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _get_flag_value_from_var(flag, var, delim=' '):
 """ 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: '/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. """


    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:]