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
diff --git a/include/atomic b/include/atomic
index 91f1829..b01a59f 100644
--- a/include/atomic
+++ b/include/atomic
@@ -533,6 +533,10 @@
 #pragma GCC system_header
 #endif
 
+#ifdef _LIBCPP_HAS_NO_THREADS
+#error <atomic> is not supported on this single threaded system
+#else // !_LIBCPP_HAS_NO_THREADS
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if !__has_feature(cxx_atomic) && _GNUC_VER < 407
@@ -1779,4 +1783,6 @@
 
 _LIBCPP_END_NAMESPACE_STD
 
+#endif  // !_LIBCPP_HAS_NO_THREADS
+
 #endif  // _LIBCPP_ATOMIC