Reintroduce check for missing binary, using a different method.

Another way to do this is to create a target, but when I tried that
it reintroduced the dependency analysis problem-- rules with this
dependency always triggered. Of course I might have missed something
subtle.  Both the target approach and this approach come from
http://stackoverflow.com/questions/5618615/check-if-a-program-exists-from-a-makefile)

This always runs, but unless you're just running clean you'd always want to
perform this check, so I think that's ok.

Makefile

@@ -84,6 +84,9 @@ EMOJI_BUILDER = third_party/color_emoji/emoji_builder.py
 ADD_GLYPHS = third_party/color_emoji/add_glyphs.py
 PUA_ADDER = map_pua_emoji.py
 VS_ADDER = add_vs_cmap.py
+ifeq (, $(shell which $(VS_ADDER)))
+  $(error "$(VS_ADDER) not in path, run setup.py in nototools")
+endif
 
 %.ttx: %.ttx.tmpl $(ADD_GLYPHS) $(UNI) $(PNG128_FLAGS)
 	python $(ADD_GLYPHS) "$<" "$@" "$(EMOJI_PNG128)"