background: Use AM_CPPFLAGS rather than INCLUDES

This is deprecated in newer automake versions, and this causes warnings
with automake 1.14:
panels/printers/Makefile.am:3: warning: 'INCLUDES' is the old name for
AM_CPPFLAGS' (or '*_CPPFLAGS')

https://bugzilla.gnome.org/show_bug.cgi?id=732189

panels/background/Makefile.am

@@ -7,7 +7,7 @@ dist_slideshowicon_DATA = slideshow-symbolic.svg
 slideshowemblemdir = $(datadir)/icons/hicolor/scalable/emblems/
 dist_slideshowemblem_DATA = slideshow-emblem.svg
 
-INCLUDES =						\
+AM_CPPFLAGS =						\
 	$(PANEL_CFLAGS)					\
 	$(BACKGROUND_PANEL_CFLAGS)			\
 	-DGNOMELOCALEDIR="\"$(datadir)/locale\""	\