• Update to version 2.2.14
• Register missing dependencies found by stage-qa
• Add LICENSE_FILE
• Use PLIST_SUB for library version

Changelog:

Provide Cgu::AsyncChannel class (c++-gtk-utils/Makefile.am;
doxygen.main.in; async_channel.h, async_queue.h;
tests/Makefile.am, tests/test_async_channel.cpp).

Remove the 'noexcept' specifier for Cgu::Thread::Cond::wait() and
Cgu::Thread::Cond::timed_wait(): any cancellation pseudo-exception
is inconsistent with this specifier (mutex.h).

Provide TaskManager::get_max_tasks() method with documentation on
rate limiting (parallel.h, task_manager.h, task_manager.cpp).

Zero initialise the character buffer in Reassembler class
(reassembler.h).

Adjust configuration macros to enable use of Cgu::Extension with
guile-2.2 (acinclude.m4; cgu_config.h.in, extension.h).

Increase coverage of io_watch tests (tests/test_iowatch.cpp).

Update documentation (async_queue.h, async_result.h, future.h,
task_manager.h, thread.h).