gnulib-common.m4: port _Noreturn to C++ Problem reported by Akim Demaille in: https://lists.gnu.org/r/bug-bison/2018-10/msg00067.html * m4/gnulib-common.m4 (gl_COMMON_BODY): If C++, use [[noreturn]]. Merge adjustments from _Noreturn.h and from glibc into the non-C++ version. * lib/_Noreturn.h: Match gnulib-common.