There is a problem with 7674543f, in that if HTML_FILE_EXTENSION or
MAN_EXTENSION is not specified, we'll try to concatenate a string with
None, and get a type error. We should check that they are set before
trying to use them.
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.
