Problem reported by Jeffery Walton in:
https://lists.gnu.org/r/bug-gnulib/2020-02/msg00013.html
* lib/_Noreturn.h (_Noreturn):
* m4/gnulib-common.m4 (gl_COMMON_BODY):
Assume _Noreturn works in Clang 3.5 and later.
It is documented to work in Clang 3.5:
http://releases.llvm.org/3.5.0/tools/clang/docs/AttributeReference.html
and is not documented in Clang 3.4:
https://releases.llvm.org/3.4/tools/clang/docs/LanguageExtensions.html
Apple sets __clang_version__ to a different value, so use
__apple_build_version__ there. See:
https://lists.gnu.org/r/bug-gnulib/2020-02/msg00017.html