Add intermediate "autoconf" target to debian/rules.

This keeps the information in a central place, and resolves LP:#854868,
where copying the ChangeLog was missing.

README

@@ -7,9 +7,7 @@ install locally, using the following instructions...
 
  0) If you have pulled the source from the upstream bzr repository
     (bzr branch lp:byobu) then you first need to run:
-      aclocal
-      automake --add-missing --copy
-      autoconf
+      ./debian/rules autoconf
  1) Or download the latest version (>= 3.0) from:
       https://launchpad.net/byobu/+download
  2) Extract:

debian/rules

@@ -22,12 +22,14 @@ update-pot:
 		msgmerge $${po} po/${PKG}.pot -o $${po} ; \
 	done
 
-prebuild: clean
+autoconf: clean
 	dh_testdir
 	cp debian/changelog ChangeLog
 	aclocal
-	automake --add-missing
+	automake --add-missing --copy
 	autoconf
+
+prebuild: autoconf
 	./configure --prefix=/usr
 
 build: prebuild