* cont.c: disable FIBER_USE_NATIVE on GNU/Hurd because it doesn't
  support a combination getcontext() and threads. Patch by
  Gabriele Giacone (1o5g4r8o@gmail.com). [Bug #8990][ruby-core:57685]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43193 b2dd03c8-39d4-4d8f-98ff-823fe69b080e