gc.c: prototype

* gc.c (gc_finalize_deferred_register): define in prototype style,
  instead of old K&R style.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44256 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
configure.in: move opt-dir option

* configure.in (opt-dir): move so that it can affect in configure
  not only after rbconfig.rb.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44257 b2dd03c8-39d4-4d8f-98ff-823fe69b080e