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

src/future.cpp

@@ -7,6 +7,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "__config"
+
+#ifndef _LIBCPP_HAS_NO_THREADS
+
 #include "future"
 #include "string"
 
@@ -298,3 +302,5 @@ shared_future<void>::operator=(const shared_future& __rhs)
 }
 
 _LIBCPP_END_NAMESPACE_STD
+
+#endif // !_LIBCPP_HAS_NO_THREADS