build: Update ax_cxx_compile_stdcxx macro
* 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: T6141