* thread.c (rb_thread_atfork_internal): My compiler complains
  about this variable being used before initialized. I looked at
  the code and expanded the macro and turned out it was actually
  USED for pointer arithmetic, not dereferenced.  So this was
  never a serious bug.  But is annoying indeed to see warnings
  every time.  I added `=0` and all went healthy.

* configure.in: Also, I found that the problematic macro expansion
  only happens when we lack __typeof__ C extension, which shall
  not be the case of my compiler.  I added AC_CTYPEOF to kick ass.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46278 b2dd03c8-39d4-4d8f-98ff-823fe69b080e