For the ports modules building code, clean WRKDIR before building. This is
important for those that use -DNO_CLEAN routinely, since it will prevent
installing stale stuff, and even more important when the port is upgraded
to a newer version. When the user doesn't use -DNO_CLEAN, this will create
an infinitesimal amount of extra work, but won't hurt anything.

This is necessary because the ports tree has flags that prevent the ususal
'update the build if newer source files exist' logic from doing what it
would do in the base.