Address some post-commit review comments on r217261

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@217276 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/future b/include/future
index 7cd88cb..83513fa 100644
--- a/include/future
+++ b/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