Note that I removed several #define whose purpose was to avoid typing "typename
ctraits<t>::…". Even though this tends to complicates the code, this is
probably safer, especially since the #define was capturing a free variable (t).
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).
Sébastien Villemot