diff -rU4 disper-orig/Makefile disper/Makefile
--- disper-orig/Makefile	2011-09-09 16:27:16.000000000 +0200
+++ disper/Makefile	2014-09-17 20:54:48.310555843 +0200
@@ -1,9 +1,17 @@
 # simple Makefile for disper
 
-PREFIX = /usr/local
-BINDIR = $(PREFIX)/bin
-DATADIR = $(PREFIX)/share/disper
+$(if $(strip $(PREFIX)),,$(PREFIX=/usr/local))
+$(if $(strip $(BINDIR)),,$(BINDIR=$(PREFIX)/bin))
+$(if $(strip $(DATADIR)),,$(DATADIR=$(PREFIX)/share))
+
+$(warning $(value $PREFIX))
+$(warning $(value BINDIR))
+$(warning $(value DATADIR))
+
+DATADIR := $(join $(DATADIR), /disper)
+
+$(warning $(value DATADIR))
 
 INSTALL = install
 
 all: disper disper.1