Log Message:
Fix ruby-build-ri-db option related problem.

- Include options.mk before rubyversion.mk, so PLIST for ri database
  should be created suitably.

- make RUBY_RIDIR and its friends relative path to ${PREFIX}.

- Fix and improve handling of ${RUBY_RIDIR} handling and should
  be fixed remaining ${RUBY_RIDIR} after pkg_delete ruby18-base.
  (Noted by private mail from wiz@.)

Bump PKGREVISION of ruby18-base package.