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

include/condition_variable

@@ -115,6 +115,8 @@ public:
 #pragma GCC system_header
 #endif
 
+#ifndef _LIBCPP_HAS_NO_THREADS
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 class _LIBCPP_TYPE_VIS condition_variable_any
@@ -253,4 +255,6 @@ void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);
 
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // !_LIBCPP_HAS_NO_THREADS
+
 #endif  // _LIBCPP_CONDITION_VARIABLE