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

llvm-svn: 217271
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: b3fcc67f8f13cd95d36ed29d0ae1308decb9e099
diff --git a/src/future.cpp b/src/future.cpp
index c67dc58..0c5c2c4 100644
--- a/src/future.cpp
+++ b/src/future.cpp
@@ -7,6 +7,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "__config"
+
+#ifndef _LIBCPP_HAS_NO_THREADS
+
 #include "future"
 #include "string"
 
@@ -298,3 +302,5 @@
 }
 
 _LIBCPP_END_NAMESPACE_STD
+
+#endif // !_LIBCPP_HAS_NO_THREADS