libpng: fix Makefile.autotools.in fallout

libpng used to have the 'png' Makefile alias, which some packages used
in their dependencies list.

With the move to Makefile.autotools.in this is now gone, so update the
packages to match.

package/cairo/cairo.mk

@@ -46,6 +46,6 @@ ifeq ($(BR2_PACKAGE_DIRECTFB),y)
 	CAIRO_DEPENDENCIES_EXTRA = directfb
 endif
 
-CAIRO_DEPENDENCIES = uclibc gettext libintl pkgconfig libglib2 zlib png fontconfig $(CAIRO_DEPENDENCIES_EXTRA) pixman $(XSERVER) 
+CAIRO_DEPENDENCIES = uclibc gettext libintl pkgconfig libglib2 zlib libpng fontconfig $(CAIRO_DEPENDENCIES_EXTRA) pixman $(XSERVER) 
 
 $(eval $(call AUTOTARGETS,package,cairo))

package/java/classpath/classpath.mk

@@ -53,7 +53,7 @@ CLASSPATH_CONF_OPT = \
 		--disable-debug \
 		--disable-gconf-peer --disable-examples --disable-plugin
 
-CLASSPATH_DEPENDENCIES = uclibc pkgconfig png jpeg tiff
+CLASSPATH_DEPENDENCIES = uclibc pkgconfig libpng jpeg tiff
 
 ifeq ($(BR2_PACKAGE_ALSA_LIB),y)
 	CLASSPATH_DEPENDENCIES+= alsa-lib

package/libgtk2/libgtk2.mk

@@ -111,6 +111,6 @@ else
 	LIBGTK2_CONF_OPT += --without-x
 endif
 
-LIBGTK2_DEPENDENCIES = uclibc pkgconfig png jpeg tiff $(LIBGTK2_DEPENDENCIES_EXTRA) libglib2 cairo pango atk
+LIBGTK2_DEPENDENCIES = uclibc pkgconfig libpng jpeg tiff $(LIBGTK2_DEPENDENCIES_EXTRA) libglib2 cairo pango atk
 
 $(eval $(call AUTOTARGETS,package,libgtk2))