merge revision(s) 43179,43180,43181,43192,43193: [Backport #8990] [Backport #9000]

	* configure.in, win32/Makefile.sub (FIBER_USE_NATIVE): move conditions
	  from cont.c

	* cont.c (FIBER_USE_NATIVE): split long conditions.

	* 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/branches/ruby_2_0_0@43226 b2dd03c8-39d4-4d8f-98ff-823fe69b080e