lang/ruby: add handling for ruby-manpages

1. Introduce RUBYGEM_MANPAGES

ruby-manpages pacakge is a plugin for rubygems and it changes behavior of
gem based pacakges (installing man pages to PKGMANDIR or not).

If ruby gem based package contains manual pages, it might fail to build
depending on installation of ruby-manpages.

So, introduce RUBYGEM_MANPAGES which mandate installation of ruby-manpages
packages or not.  RUBYGEM_MANPAGES should be set to "YES" ruby gem based
package contains manual pages.

2. Remove obosolete comment lines.

(taca)