according to https://www.gnu.org/prep/standards/html_node/Standard-Targets.html#Standard-Targets
`maintainer-clean` is the proper make target for files that are distributed
in a release tarball:
> The ‘maintainer-clean’ target is intended to be used by a maintainer of the
> package, not by ordinary users.
> You may need special tools to reconstruct some of the files that
> ‘make maintainer-clean’ deletes.
this prevents users without a2x or asciidoctor from losing their ability to
recompile tinyproxy after `make clean`, but it also means that users wanting
to regenerate the documentation need to run `make maintainer-clean`.
asciidoctor is a modern replacement for asciidoc and much more lightweight,
issuing "apt-get install asciidoc" on ubuntu 16.04 results in an attempt to
install more than 1.3 GB of dependencies.
asciidoc is necessary as the version number is added during
configure into the asciidoc manpage sources. So simply bundling
a pre-generated manpage won't do.
