This issue was solved with changeset r34448.
Yuki, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.
merge revision(s) 33886: [Backport #5682]
* common.mk (INSTRUBY_ARGS): added --mantype to apply mdoc2man.rb
to man pages. Fixes #5598.
(do-install-nodoc, do-install-local, do-install-man,
dont-install-nodoc, dont-install-local, dont-install-man):
No longer needs --mantype.
Reported by Rainer Orth <ro AT cebitec.uni-bielefeld.de>,
patch by George Koehler <xkernigh AT netscape.net>.