Problem reported by Joe Nelson in:
https://lists.gnu.org/r/bug-gnulib/2021-01/msg00152.html
* doc/noreturn.texi: Improve.
* lib/_Noreturn.h (_Noreturn):
* m4/gnulib-common.m4 (gl_COMMON_BODY):
Do not assume _Noreturn works as-is when __STRICT_ANSI__ is
defined, unless __STDC_VERSION__ indicates C11 or later.
* lib/_Noreturn.h (_Noreturn): Fall back on __attribute__
((__noreturn__)) if Clang; this merges the
2020-08-10T23:53:13Zbruno@clisp.org patch to m4/gnulib-common.m4.