configure.in: fix cross-compiling installation failure

* configure.in (RUNRUBY): append -- only after runruby.rb, not
  cross-compiling baseruby, so that $(RUN_OPT) can be command line
  options.  [ruby-dev:47703] [Bug #8893]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42910 b2dd03c8-39d4-4d8f-98ff-823fe69b080e