avoid usage of Macport installed software

[petr]

Hi all,

I am working on some new port. It works already quite fine, but I observe that when using it without Tracemode, it would pick up various already installed programs. Most of these are also provided by Xcode, but notably when `doxygen` is picked up, it also creates some extra documentation. This all works smoothly, but it would not create a consistent build across various installs. Is there a canonical way to avoid such problems?

Thanks!
~petr