docs: build and install tar ball only for the latest tag, also clean tar balls

documentation/Makefile

@@ -82,7 +82,9 @@ RELEASE_TAGS := $(shell darcs show tags | egrep '^([R\-][0-9]+)+$$')
 RELEASES := $(call tag2rel,$(RELEASE_TAGS))
 TARS := $(RELEASES:%=$(PROJECT)-%.tar.gz)
 
-tar: $(TARS)
+CLEANS += $(TARS)
+
+tar: $(firstword $(TARS))
 
 $(PROJECT)-%.tar.gz:
 	darcs dist -d documentation/$(PROJECT)-$* -t $(call rel2tag,$*) $(SUPPRESS_OUTPUT)