proper DOXYGEN option default value

[Don Lewis]

I maintain a number of ports that use doxygen to generate their
documentation.  They all list DOXYGEN in OPTIONS_DEFAULT so that
the packages will include documentation.

I recently received a PR for one of those ports that mentioned that most
users don't require the documentation and pointed out that anyone
building the port will find it is very painful because of the amount of
time it takes to build doxygen and all of its dependencies (primarily
all the TeX stuff).

It's been my experience that there is enough churn in the dependencies
that the whole mess needs to get rebuilt fairly frequently.

As I see it, the pros and cons of listing DOXYGEN in OPTIONS_DEFAULT
are:
  Pros:
    * Users who need the documentation can use the binary packages if
      they desire
    * This option gets regular exercise on the build cluster
  Cons:
      * A bit of extra load on the ports build cluster to install
        doxygen and its dependencies as a BUILD_DEPENDS for each
        supported OS version and architecture
      * The packages are a bit larger
      * A bit more space is consumed on the user's machine because
        documentation is installed, even if they don't need it
      * Users building from ports have to build and install doxygen
        and it's dependencies unless they take measures to avoid it,
        either by disabling the option for each port or adding
        OPTIONS_UNSET=DOXYGEN to their /etc/make.conf to do this
        globally

The pros and cons of removing DOXYGEN from OPTIONS_DEFAULT:
  Pros:
     * Less load on the ports build cluster
     * Smaller packages
     * Less space consumed on most user machines
     * Users building from ports who do not need the documentation don't
       pay the doxygen penalty without taking any special measures
  Cons:
     * Anyone who does need the documentation has to build from the
       port and pay the cost of building and installing doxygen and its
       dependencies.
     * The DOXYGEN option would not get regular testing and could be
       subject to bit rot.

Ideally, the documentation could be built and packaged separately.  It
could even be NO_ARCH.   Unfortunately, at this time this would require
adding a bunch of new ports just for the documentation.

Since these ports only generate HTML documentation, it might be helpful
to have a doxygen-lite port that has GRAPHVIZ and LATEX disabled, and
use that as a BUILD_DEPENDS.  That introduces the problem of conflicts
between doxygen and doxygen-lite if someone who builds from ports needs
the full featured doxygen for other purposes.

Since we have neither of the above, should I leave my ports as-is, or
should I disable DOXYGEN by default?