merge revision(s) 39775: [Backport #8080]

	* thread.c: disabled _FORTIFY_SOURCE for avoid to hit glibc bug.
	  [Bug #8080] [ruby-core:53349]

	* test/ruby/test_io.rb (TestIO#test_io_select_with_many_files):
	  test for the above.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@39840 b2dd03c8-39d4-4d8f-98ff-823fe69b080e