Dynare++: use C++11 threads (instead of POSIX threads API)

On Windows, this means that a POSIX threads implementation is no longer needed,
since C++11 threads are implemented using native Windows threads.

On GNU/Linux and macOS, POSIX threads are still used under the hood.

A new m4 macro (AX_CXX11_THREAD) is used to add the proper compilation
flags (instead of AX_PTHREAD).