If the port requires user input to build, configure, or
      install, set IS_INTERACTIVE in the
      Makefile.  This will allow
      “overnight builds” to skip it.  If the user
      sets the variable BATCH in their environment (and
      if the user sets the variable INTERACTIVE, then
      only those ports requiring interaction are
      built).  This will save a lot of wasted time on the set of
      machines that continually build ports (see below).
It is also recommended that if there are reasonable default
      answers to the questions,
      PACKAGE_BUILDING be used to turn off the
      interactive script when it is set.  This will allow us to build
      the packages for CDROMs and FTP.
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