They're based on obsolete mkit.ini structure and is not used in any known
project.  That said, at least build_manpages will be added in future,
most probably in form of plugin, once plugin structure is figured out.
(Stay tuned, but don't hold your breath!)

Found by running ShellCheck on .mk file ... by accident!

Usage of GNU Make really is just a sugar, so we should enable all
targets also for direct mkit/make invocation.

DESTDIR[1] can be now set to specify the root in situations such as when
building a package or installing to a chrooted environment.

If not specified by environment variable (eg. by make) or in mkit.ini,
the default value is governed by MKIT_DEFAULT_DESTDIR, which defaults to
"/".

  [1]: https://www.gnu.org/prep/standards/html_node/DESTDIR.html