Revert recent man/Makefile.am change.

Revert recent man/Makefile.am change.

Revert 4b544e447eb78fd1f031a026a499f6aed177808a.
* man/Makefile.am: That change was only a band-aid.  It solved
the stated problem, but not a deeper one: that "make install"
would no longer man pages.  Reported by Dmitry V. Levin.
The latter problem arose in
167b8025aca487de001da2448c1aebc2747bc1d3 with the removal of
the sole literal from the definition of dist_man_MANS.  When
automake perceives dist_man_MANS as empty, it no longer emits
the install-man* rules.