merge revision(s) 39958,39989: [Backport #8169]

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

	* 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/branches/ruby_2_0_0@40289 b2dd03c8-39d4-4d8f-98ff-823fe69b080e