It is set up to put libnet.pdf in $(docdir) at "make install" time, and to build it at "make" time.
*If* pdf generation is enabled.
If not, it is not generated at all, of course.
It isn't put in the "make dist" tarball, because it seems like the contents of that
should be invariant to configuration parameters.

Similarly with the man files, although the case there is not so strong.

(I'd think that "make dist" should work even if doxygen is completely unavailable.)