Make DESTDIR overridable

Allow the user to set the installation root as an alternative to "/".

Contributed by Per Olav Kroka.

Signed-off-by: Jean Delvare <jdelvare@suse.de>

Makefile

@@ -7,7 +7,7 @@
 # the Free Software Foundation; either version 2 of the License, or
 # (at your option) any later version.
 
-DESTDIR	=
+DESTDIR	?=
 prefix	= /usr/local
 bindir	= $(prefix)/bin
 sbindir	= $(prefix)/sbin