[v3 PATCH] Protect optional's deduction guide with the feature macro

[Ville Voutilainen]

Tested partially on Linux-x64, finishing testing the full suite on
Linux-PPC64. Ok for trunk?

2018-01-03  Ville Voutilainen  <ville.voutilainen@gmail.com>

    Protect optional's deduction guide with the feature macro
    * include/std/optional: Use the feature macro.
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diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index e017eed..f7c72b5 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -1038,7 +1038,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   /// @}
 
+#if __cpp_deduction_guides >= 201606
   template <typename _Tp> optional(_Tp) -> optional<_Tp>;
+#endif
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std