* m4/stdbool.m4 (AC_CHECK_HEADER_STDBOOL): Copy from bleeding-edge
Autoconf. Don’t define if Autoconf 2.72 or later.
* tests/test-stdbool.c (bool, false, true)
(__bool_true_false_are_defined): Do not require these to be
defined. C23 still requires __bool_true_false_are_defined but
marks it as obsolescent, and it’s no longer worth testing.
(WORKING_BOOL): New macro, which also tests __STDC_VERSION__.
Use it in tests for working bool.
Test for bool instead of for _Bool, which C23 says is obsolescent.