* m4/ax_cxx_compile_stdcxx.m4: Replace with current version from the
autoconf archive.
--
This fixes the problem that the switch -std=c++11 was omitted if the
compiler supported C++11 features by default. This made gcc happily
compile C++14 code. Now C++11 is enforced by gcc.
GnuPG-bug-id: 6141