Backport gcc4 compiler fix to v1.0.0

[leezu]

Change the initialization for default constructor to avoid using explicit constructor (#8854)

See #8169

[cbalioglu]

Why not just
template<> std::unordered_set<std::string> OperatorTune<__typ$>::operator_names_{} (C++11)
or
template<> std::unordered_set<std::string> OperatorTune<__typ$>::operator_names_() (C++98)?

[cjolivier01]

I've had similar problems with some versions of gcc. I found ({}) to be the best way in operator_tun.cc -- I remember trying other variations, but sometimes it wouldn't create the static variable at all.