Adds Link Time Optimization
Adds Link Time Optimization

It also adds the --no-undefined linker flag.
This is a port of  Hong Jen Yee (PCMan) work on LXQtCompilerSettings.

libqtxdg/libqtxdg,luis-pereira/libqtxdg,libqtxdg/libqtxdg,lxde/libqtxdg,luis-pereira/libqtxdg,lxde/libqtxdg