Address some post-commit review comments on r217261

llvm-svn: 217276
diff --git a/libcxx/include/future b/libcxx/include/future
index 7cd88cb..83513fa 100644
--- a/libcxx/include/future
+++ b/libcxx/include/future
@@ -374,7 +374,7 @@
 #pragma GCC system_header
 #endif
 
-#ifndef _LIBCPP_HAS_NO_THREADS
+#ifdef _LIBCPP_HAS_NO_THREADS
 #error <future> is not supported on this single threaded system
 #else // !_LIBCPP_HAS_NO_THREADS