This will allow std::mutex and std::lock_guard to be used elsewhere in
the library without pulling in the whole of <chrono>.

Previously the whole of <bits/std_mutex.h> was conditional on the
_GLIBCXX_USE_C99_STDINT_TR1 macro, but only the std::unique_lock members
that use <chrono> facilities should depend on that. std::mutex only
needs to depend on _GLIBCXX_HAS_GTHREADS and std::lock_guard can be
defined unconditionally.

Some parts of <bits/std_mutex.h> and <mutex> are based on code in
<ext/concurrence.h> which dates from 2003. However, the std::unique_lock
implementation was added in 2008 by r135007, without using any earlier
code. Therefore the new header file has copyright years 2008-2018.

	* include/Makefile.am: Add new <bits/unique_lock.h> header.
	* include/Makefile.in: Regenerate.
	* include/bits/std_mutex.h [!_GLIBCXX_USE_C99_STDINT_TR1] (mutex)
	(lock_guard): Define independent of _GLIBCXX_USE_C99_STDINT_TR1.
	(unique_lock): Move definition to ...
	* include/bits/unique_lock.h: New header.
	[!_GLIBCXX_USE_C99_STDINT_TR1] (unique_lock): Define unconditionally.
	[_GLIBCXX_USE_C99_STDINT_TR1] (unique_lock(mutex_type&, time_point))
	(unique_lock(mutex_type&, duration), unique_lock::try_lock_until)
	(unique_lock::try_lock_for): Define only when <chrono> is usable.
	* include/std/condition_variable: Include <bits/unique_lock.h>.
	* include/std/mutex: Likewise.

From-SVN: r262963