--- Makefile.orig	2016-01-19 16:18:43 UTC
+++ Makefile
@@ -4,7 +4,7 @@ GPRBUILD  = gprbuild
 GPRCLEAN = gprclean
 GPRINSTALL = gprinstall
 
-INSTALL:=$(shell exec=`which gprbuild`;if [ ! -x "$$exec" ]; then unset exec;fi;echo $$exec | sed -e 's/\/bin\/$(GPRBUILD).*//')
+INSTALL:=$(DESTDIR)$(PREFIX)
 
 ifeq ($(RTS),)
    RTS=full