Reported by Pierre Ossman <ossman@cendio.se>
at <https://savannah.gnu.org/bugs/?67152>.
* m4/stddef_h.m4 (gl_STDDEF_H): Also test whether unreachable is defined
by <stddef.h> in C++ mode.
* lib/stddef.in.h: In C++ mode, include <utility> and either import
'unreachable' from the std namespace or define it as an inline function.
* tests/test-stddef-h.c: Disable some tests in C++ mode.
* tests/test-stddef-h-c++.cc: Perform nearly the same tests in C++ mode
as in C mode.
* tests/test-stddef-h-c++2.cc: Rename some variables. Disable the NULL
test with clang on Windows.
* tests/test-stddef-h-c++3.cc: New file.
* modules/stddef-h-c++-tests (Files): Include it.
(Makefile.am): Link test-stddef-h-c++ with test-stddef-h-c++3.o.