Add a DESTDIR variable to all Makefiles, defaulting to the empty string, and use it in install targets. See xemacs-patches message <BANLkTinRnwk9qsCfBj_xkcyf7H4sH9fn7A@mail.gmail.com>.

#!/bin/sh
if
test -r $2
then
if
cmp $1 $2 > /dev/null
then
echo $2 is unchanged
rm -f $1
else
mv -f $1 $2
fi
else
mv -f $1 $2
fi