Makefile: avoid having vignettes/chartOpts.Rmd re-built every time

 - Add ".PHONY:" to indicate phony targets
 - Don't have files depend on those phony targets but instead define
   variables that list the real targets and then define the phony
   targets using those variables.
Makefile: a bit of rearrangement, and split a long line
Use html comment code on %\Vignette lines