Update to version 2.2.12

Changelog:
Simplify Cgu::Application::add() method (application.cpp).

Minor implementation improvement for
Cgu::Thread::parallel_transform* functions (do pointer arithmetic
before not after tasks are launched) (parallel.h).

Permit return value optimisation when returning thread object on
starting new thread (thread.cpp).

Update libtool to version 2.4.6.

Documentation fix for doxygen-1.8.9 (docs/Doxyfile.in).

Update code comments (emitter.h).

Update documentation (doxygen.main.in, application.h, emitter.h,
extension.h, future.h, future.tpp, shared_handle.h, shared_ptr.h,
task_manager.h, thread.h).