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/__sso_allocator b/include/__sso_allocator
index d25fc48..16354d8 100644
--- a/include/__sso_allocator
+++ b/include/__sso_allocator
@@ -15,7 +15,9 @@
 #include <type_traits>
 #include <new>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD