libstdc++: Fix build failure for target with no way to sleep

In previous releases the std::this_thread::sleep_for function was only
declared if the target supports multiple threads. I changed that
recently in r11-2649-g5bbb1f3000c57fd4d95969b30fa0e35be6d54ffb so that
sleep_for could be used single-threaded. But that means that targets
using --disable-threads are now required to provide some way to sleep.
This breaks the build for (at least) AVR when trying to build a hosted
library.

This patch adds a new autoconf macro that is defined when no way to
sleep is available, and uses that to suppress the sleeping functions in
std::this_thread.

The #error in src/c++11/thread.cc is retained for the case where there
is no sleep function available but multiple threads are supported. This
is consistent with previous releases, but that #error could probably be
removed without any consequences.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_ENABLE_LIBSTDCXX_TIME): Define NO_SLEEP
	if none of nanosleep, sleep and Sleep is available.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* include/std/thread [_GLIBCXX_NO_SLEEP] (__sleep_for): Do
	not declare.
	[_GLIBCXX_NO_SLEEP] (sleep_for, sleep_until): Do not
	define.
	* src/c++11/thread.cc [_GLIBCXX_NO_SLEEP] (__sleep_for): Do
	not define.