snippet/_Noreturn: port to pedantic clang 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.