[PATCH] [libc++] Allow libc++ to be built on systems without POSIX threads

[Jon Roelofs]

================
Comment at: sources/cxx-stl/llvm-libc++/libcxx/include/__config:14
@@ -13,1 +13,3 @@
 
+#include <unistd.h>
+
----------------
awong also suggested that I wrap this in an `#if !_WIN32`.

http://reviews.llvm.org/D3969