Makefile: allow set DISTS and VERSION

Makefile

@@ -1,5 +1,5 @@
-DISTS:=buster jessie stretch wheezy precise trusty xenial
-VERSION:=5.1.1
+DISTS ?= buster jessie stretch wheezy precise trusty xenial
+VERSION ?= 5.1.6
 
 all: clean
 	for i in $(DISTS) ; do \