mkmf.rb: force refererence in MAIN_DOES_NOTHING

* lib/mkmf.rb (MAIN_DOES_NOTHING): ensure symbols for tests to be
  preserved.  [ruby-core:53745] [Bug #8169]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39989 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-03-29

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39990 b2dd03c8-39d4-4d8f-98ff-823fe69b080e