If a primary screen is set and the CRTC information can be retrieved,
i3-nagbar will now open on the primary screen. Otherwise, the old (fixed)
position will be used as a fallback.
fixes#1405
CPPFLGES, CFLAGS and LDFLAGS should be user variables
We now provide default flags but use I3_*FLAGS flags for our own needed
flags
Also reoder lib flags a bit