Log Message:
Update to 2.2.15

Changelog:
Version 2.2.15 (24 December 2017)
--------------

	Provide configuration option for GTK+-4 (acinclude.m4,
	bootstrap-gtk2.sh, bootstrap-gtk3.sh, bootstrap-gtk4.sh,
	configure-gtk4.ac, Makefile.am, README; doxygen.main.in,
	file_print_manager.cpp, text_print_manager.cpp, window.h;
	tests/test_application.cpp;
	tests/extra/test_file_print_manager.cpp,
	tests/extra/test_text_print_manager.cpp).

	Use streambuffers not streams when testing fdstream and gstream
	with char16_t and char32_t character types
	(tests/test_fdstream.cpp, tests/test_gstream.cpp).

	Update documentation (doxygen.main.in; async_channel.h,
	callback.h, fdstream.h, gstream.h, task_manager.h,
	task_manager.cpp).

Version 2.2.14.1 (23 November 2016)
----------------

	Explicitly qualify malloc() as being within the std namespace
	(async_channel.h).

	Do not document the Cgu::Thread::FutureHelper and
	Cgu::Thread::TaskManagerHelper2 namespaces (future.h,
	task_manager.h).

	Update documentation (async_channel.h).

Version 2.2.14 (9 November 2016)
--------------

	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).