use <db.h> before <db3/db.h> or <db4/db.h>

use <db.h> before <db3/db.h> or <db4/db.h>
this is the only way i can think of to make it work everywhere; people on
systems like FreeBSD where the ports work the other way around will have to
specify an extra -I flag in their CPPFLAGS variable.