Set output_formats["MAN"] to "NO", by default, as suggested at
http://www.scons.org/wiki/DoxygenBuilder#Note_added_by_Reinderien. Despite
what the Doxygen docs say, GENERATE_MAN is off by default, and a
browse of the source suggests that it's been that way since at least
2008 / Doxygen 1.5.7.