Add screenshots and defs to the dependencies

... not that I'm actually using them for the time being ...

GNUmakefile

@@ -5,8 +5,12 @@ TOPLEVEL=apt-manual
 INSTALL=boss.emulab.net:/usr/testbed/devel/ricci/
 
 SCRIBBLEFILES=$(wildcard *.scrbl) 
+SCREENSHOTFILES=$(wildcard screenshots/*)
+DEFSFILE=defs.rkt
 
-all: $(SCRIBBLEFILES)
+SOURCES=$(SCRIBBLEFILES) $(SCREENSHOTFILES) $(DEFSFILE)
+
+all: $(SOURCES)
 	scribble --htmls $(TOPLEVEL).scrbl
 
 clean: