Add a new target "make update-png" instead of regenerating the PNG files

automatically as SVN doesn't preserve the mtime of the file, some files 
where generated on the build system where rsvg might not be available, 
or available in a different version producing slightly different output, 
thus causing visual discrepancies in the resulting theme...


(Old svn revision: 23778)

themes/default/Makefile.am

@@ -174,5 +174,9 @@ noinst_DATA =								\
 	$(SVG_FILES)							\
 	vaccumdefs.sh
 
-.svg.png:
-	rsvg -f png  $< $@
+update-png:
+	for svg_file in $(SVG_FILES); do \
+		image=$${svg_file%.svg}; \
+		echo "Converting "$$svg_file" to "$$image.png" ..."; \
+		rsvg -f png $$svg_file $$image.png; \
+	done