build: end all multiline list variables with $(NULL)

So that appending a new item in the list only inserts one new line (i.e. the
last $(NULL) is the last item always).

Makefile.am

@@ -1,27 +1,49 @@
 
-SUBDIRS = . build-aux data include libqcdm libwmc libmm-glib src plugins cli vapi introspection uml290 po test examples docs
+SUBDIRS = \
+	. \
+	build-aux \
+	data \
+	include \
+	libqcdm \
+	libwmc \
+	libmm-glib \
+	src \
+	plugins \
+	cli \
+	vapi \
+	introspection \
+	uml290 \
+	po \
+	test \
+	examples \
+	docs \
+	$(NULL)
 
 DISTCHECK_CONFIGURE_FLAGS = \
 	--with-udev-base-dir="$$dc_install_base" \
 	--with-systemdsystemunitdir="$$dc_install_base/$(SYSTEMD_UNIT_DIR)" \
-	--enable-gtk-doc=yes
+	--enable-gtk-doc=yes \
+	$(NULL)
 
 INTLTOOL_FILES = \
 	intltool-extract.in \
 	intltool-merge.in \
-	intltool-update.in
+	intltool-update.in \
+	$(NULL)
 
 DISTCLEANFILES = \
 	intltool-extract \
 	intltool-merge \
 	intltool-update \
-	po/.intltool-merge-cache
+	po/.intltool-merge-cache \
+	$(NULL)
 
 EXTRA_DIST = \
 	$(INTLTOOL_FILES) \
 	autogen.sh \
 	gtester.make \
-	COPYING.LIB
+	COPYING.LIB \
+	$(NULL)
 
 ACLOCAL_AMFLAGS = -I m4