Qt Linking with X11?

Dated a year ago to get it off the front page.

Qt Application on Linux Mint trying to link with X11 libraries. Original errors are:

So it looks like it can’t find XFlush. Maybe can’t find libX11.so. Added this to the project file:

But still got this error:

But libX11.so is clearly present on system:

So how do I tell Qt to find libX11.so and link with it?

Thanks.