Re-introduce lost functionality

The branch merged in 5a863589b4 removed fine-grained configure-time control
over install paths. This is necessary to accomodate various linux distros'
policies, eg. Gentoo wants games installed in /usr/games, but with resource
files in /usr/share/games.

DOCDIR and MANDIR appear to be unused, and were not re-introduced.
cleanup

-   drop trailing slashes from paths for consistency
-   sort entries that got unsorted