fix `make vacuum` if no `_CoqProject` is present

I was repeatedly facing this situation:

	% make vacuum
	make: *** No rule to make target `_CoqProject', needed by `Makefile.coq'.  Stop.

This patch makes `vacuum` depend on `allCoqProject`, which should
ensure that we get rid of everything.

Makefile

@@ -71,7 +71,7 @@ install html gallinahtml htmlpretty clean cleanall validate alectryon: $(COQ_MAK
 %.vo: %.v
 	$(MAKE) -f $(COQ_MAKEFILE) $@
 
-vacuum: cleanall
+vacuum: allCoqProject cleanall
 	@echo 'VACUUMING *.vo *.glob .*.aux <empty directories>'
 	@find . -depth \( -iname '*.vo' -or  -iname '*.glob' -or -iname '.*.aux' \)  ! -path './.git/*' -delete
 	@find . -depth -type d -empty ! -path './.git/*' -delete