[PATCH] Use $DESTDIR instead of $dest

[PATCH] Use $DESTDIR instead of $dest

$DESTDIR is more usual during the build than $dest and is what
is usually used in the makefiles, so let's use it too.

Signed-off-by: Petr Baudis <pasky@ucw.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>