Add support for DESTDIR to "make install"

per the Debian New Maintainers' Guide:
http://www.debian.org/doc/manuals/maint-guide/modify.en.html#id459110

Author: killerswan

Makefile.in

@@ -493,6 +493,12 @@ ifneq ($(findstring clean,$(MAKECMDGOALS)),)
 endif
 
 ifneq ($(findstring install,$(MAKECMDGOALS)),)
+  ifdef DESTDIR
+    CFG_INFO := $(info cfg: setting CFG_PREFIX via DESTDIR, $(DESTDIR))
+    CFG_PREFIX:=$(DESTDIR)
+    export CFG_PREFIX
+  endif
+
   CFG_INFO := $(info cfg: including install rules)
   include $(CFG_SRC_DIR)/mk/install.mk
 endif