Windows support by Ruben Van Boxem.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@142235 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/condition_variable b/include/condition_variable
index fc3ac3d..b4da556 100644
--- a/include/condition_variable
+++ b/include/condition_variable
@@ -111,7 +111,9 @@
 #include <__mutex_base>
 #include <memory>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD