Elaborated on Fl_Cairo_Window's use of FLTK style coordinates,
and how this differs from cairo's default native normalized
coordinate system, and shows how to switch from one to the other.
Also, small comment fix to the cairo example regarding the "X" color.
Since this is the first cairo example in the examples directory,
it necessarily involved changes to the Makefile and to fltk-config
to properly handle the absence/existance of the cairo libs.
TBD: Add docs to the cario widget describing coordinate system
and how it differs from the default cairo normalized coordinate system.