+# configure touches certain files even if they haven't changed. This
+# can mean a lot of unnecessary recompilation after a re-configure, so
+# here we cache the old versions of these files so we can restore the
+# timestamps.
+%.old: %
+ @set -x && test -f $@ && cmp -s $< $@ || cp -p $< $@
+ touch -r $@ $<
+
+