Support for $DESTDIR in the Makefile.

From debian

Index: autossh-1.4e/Makefile.in
===================================================================
--- autossh-1.4e.orig/Makefile.in	2015-02-10 05:41:58.000000000 +0100
+++ autossh-1.4e/Makefile.in	2015-07-12 11:44:41.979659656 +0200
@@ -6,11 +6,11 @@
 
 SSH=		@path_ssh@
 
-prefix=		@prefix@
-exec_prefix=	@exec_prefix@
-bindir=		@bindir@
-datadir=	@datadir@
-mandir=		@mandir@
+prefix=		${DESTDIR}@prefix@
+exec_prefix=	${DESTDIR}@exec_prefix@
+bindir=		${DESTDIR}@bindir@
+datadir=	${DESTDIR}@datadir@
+mandir=		${DESTDIR}@mandir@
 
 SRCDIR=		@srcdir@
 VPATH=		@srcdir@