Makefile improvements & compiler warnings fixed

Signed-off-by: Steven Honeyman <stevenhoneyman@users.noreply.github.com>

Signed-off-by: Steven Honeyman <stevenhoneyman@users.noreply.github.com>

- allow for user specified compiler
- allow for users CXXFLAGS + LDFLAGS
- removed unnecessary implicit .o rule
- changed CC to CXX (c++ instead of c)

Signed-off-by: Steven Honeyman <stevenhoneyman@users.noreply.github.com>

Cleaned up code
Windows are now selected via EnterNotify events
Improved response time on rectangle drawing (no longer creates new rectangles when window selection changes, waits 0.01 seconds between re-drawing for xorg to catch up and display)
Removed offset settings as they seem useless
Added --version option
--nodecorations now doesn't attempt to actively search for the correct subwindow. Now it just allows for selection of subwindows. This should keep slop from crashing/not work properly on weird WMs like openbox, but may make it act a little unexpectedly since you can still click on decorations to include them.

Fix makefile issue

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.