Fix issue where librt wasn't linked in, which is sometimes required on certain systems.

It was due to the mouse in X11 selecting pixels one left and one above where you actually can put the mouse,
and doing some window border math wrong.
This caused you to be incapable of selecting the whole screen since the mouse can't position itself completely in the lower right corner,
and judging a window's reported size wrong.
I also made the tolerance option more functional; setting it to 0 will disable window selections.
Because of this I also removed the --nowindow option.

give slop's window a class

Update makefile

That's what we usually do.