only use lto on newer gcc
only use lto on newer gcc

pscedu/pfl,pscedu/pfl,pscedu/pfl,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/pfl,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable