Allow libc++ to be built on systems without POSIX threads

If you're crazy enough to want this sort of thing, then add
-D_LIBCPP_HAS_NO_THREADS to your CXXFLAGS and
--param=additiona_features=libcpp-has-no-threads to your lit commnad line.

http://reviews.llvm.org/D3969


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@217271 91177308-0d34-0410-b5e6-96231b3b80d8

include/thread

@@ -106,6 +106,10 @@ void sleep_for(const chrono::duration<Rep, Period>& rel_time);
 
 #define __STDCPP_THREADS__ __cplusplus
 
+#ifdef _LIBCPP_HAS_NO_THREADS
+#error <thread> is not supported on this single threaded system
+#else // !_LIBCPP_HAS_NO_THREADS
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp>
@@ -455,4 +459,6 @@ void yield() _NOEXCEPT {sched_yield();}
 
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // !_LIBCPP_HAS_NO_THREADS
+
 #endif  // _LIBCPP_THREAD