Avoid GNU make re-exec when adding dependencies to Makefile

GNU make will re-exec if (it thinks that) the Makefile has changed.
Just having the target Makefile seems to make it think it has, so we
end up in a look where GNU make re-execs for ever.

The fix is easy, just remove the Makefile target and have the depend
target run the recipe on its own instead of depending on Makefile.

Reviewed-by: Rich Salz <rsalz@openssl.org>