the ECHO macro is set to "echo" by default, but it is set to "true" if
make(1) is invoked with the -s option while ECHO_CMD is always set to
the echo command.
other docbook-* ports were broken in the same way, so I've taken the
liberty of updating the URL for all of them -- it looks as though OASIS
have fallen over, so point these at docbook.org for the time being.