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/shared_mutex

@@ -112,6 +112,10 @@ template <class Mutex>
 #pragma GCC system_header
 #endif
 
+#ifdef _LIBCPP_HAS_NO_THREADS
+#error <shared_mutex> is not supported on this single threaded system
+#else // !_LIBCPP_HAS_NO_THREADS
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 class _LIBCPP_TYPE_VIS shared_timed_mutex
@@ -414,6 +418,8 @@ swap(shared_lock<_Mutex>& __x, shared_lock<_Mutex>& __y) _NOEXCEPT
 
 _LIBCPP_END_NAMESPACE_STD
 
+#endif  // !_LIBCPP_HAS_NO_THREADS
+
 #endif  // _LIBCPP_STD_VER > 11
 
 #endif  // _LIBCPP_SHARED_MUTEX