Override RUBYOPT when configuring Comedilib ruby binding installation.

Override RUBYOPT when configuring Comedilib ruby binding installation.

With an unmasked RUBYOPT="-rauto_gem" [1] the .config file ignores the
DESTDIR environmental variable, for reasons I have not isolated.  This
leads to sandbox violations during `ruby setup.rb install` when the
ebuild tries to install directly to
/usr/lib/ruby/site_ruby/1.8/comedi.rb rather than
${D}/usr/lib/ruby/site_ruby/1.8/comedi.rb.

error message:
   * Call stack:
   *     ebuild.sh, line  56:  Called src_install
   *   environment, line 5179:  Called die
   * The specific snippet of code:
   *           ruby setup.rb install || die "ruby setup.rb install failed";

chmod sandbox violation:
  F: chmod
  S: deny
  P: /usr/lib/ruby/site_ruby/1.8/comedi.rb
  A: /usr/lib/ruby/site_ruby/1.8/comedi.rb
  R: /usr/lib/ruby/site_ruby/1.8/comedi.rb
  C: ruby setup.rb install
and an identical violation with `F: open_wr`.

[1] Set by dev-ruby/rubygems via /etc/env.d/10rubygems.