Support $(DESTDIR) in Makefile.

Index: bigcouch/Makefile
===================================================================
--- bigcouch.orig/Makefile	2014-01-15 16:45:52.258395008 +0000
+++ bigcouch/Makefile	2014-01-15 16:47:42.970395009 +0000
@@ -41,14 +41,10 @@
 
 include install.mk
 install: dist
-	@mkdir -p $(prefix)
-	@cp -R rel/bigcouch/* $(prefix)
-	@mkdir -p $(data_dir)
-	@chown $(user) $(data_dir)
-	@mkdir -p $(view_dir)
-	@chown $(user) $(view_dir)
-	@touch $(prefix)/var/log/bigcouch.log
-	@chown $(user) $(prefix)/var/log/bigcouch.log
+	@mkdir -p $(DESTDIR)/$(prefix)
+	@cp -R rel/bigcouch/* $(DESTDIR)/$(prefix)
+	@mkdir -p $(DESTDIR)/etc/sv
+	@cp -R rel/sv/bigcouch $(DESTDIR)/etc/sv/
 
 dev: compile
 	@rm -rf rel/dev1 rel/dev2 rel/dev3