Update to version 2.2.11

Changelog:

Provide a mutex for calls to make-fresh-user-modules to deal with
guile thread safety issues (c++-gtk-utils/Makefile.am;
extension.h, extension_helper.cpp).

Improve efficiency of scheme to C++ list translators for long
lists (extension.h).

Update libtool to version 2.4.4.

Update code comments (extension.h, future.h, parallel.h,
task_manager.h, task_manager.tpp).

Update documentation (README; application.h, parallel.h, param.h).