MP removes /tmp symlink

[markd at macports.org]

I've seen this before, but now I think I have a better idea why it
happens.  It only happens when building new portfiles when the portfiles
fail.  When files get placed in /tmp and the portfile terminates because
of errors, then the /tmp symlink gets wiped when the port is cleaned using
"port clean --all".  I know how to duplicate the problem now.

Mark