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/include/ios b/include/ios
index d95f18a..6d42607 100644
--- a/include/ios
+++ b/include/ios
@@ -216,7 +216,7 @@
 #include <__locale>
 #include <system_error>
 
-#if __has_feature(cxx_atomic)
+#if __has_feature(cxx_atomic) && !_LIBCPP_HAS_NO_THREADS
 #include <atomic>     // for __xindex_
 #endif
 
@@ -367,7 +367,7 @@
     int*            __index_;
     size_t          __event_size_;
     size_t          __event_cap_;
-#if __has_feature(cxx_atomic)
+#if __has_feature(cxx_atomic) && !_LIBCPP_HAS_NO_THREADS
     static atomic<int> __xindex_;
 #else
     static int      __xindex_;