Error in trace mode

[Piet van Oostrum]

I am testing a Portfile, and running with trace mode:

sudo port -vst install

I get this error message:

Warning: The following existing file was hidden from the build system by trace mode:
  /opt/local/bin/aclocal

aclocal is part of automake, and I have automake as build dependency:

depends_build       port:autoconf \
                    port:automake

use_autoconf        yes
autoconf.cmd        ./bootstrap

I have no idea how to get rid of this warning. Without -t the port installs without problems.
Can anyone give me an idea how to solve this?
-- 
Piet van Oostrum <piet-l at vanoostrum.org>
WWW: http://piet.vanoostrum.org/
PGP key: [8DAE142BE17999C4]