slop is now available in Void Linux

Not sure whether to link to http://repo.voidlinux.eu/current/ or the template that I constructed, which was processed by the buildbot after the merge.

Changed window clear method

Actually clear the window instead of drawing a 0x0 rectangle which for some reason pisses off compton.

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.