extmk.rb: fix extstatic

* ext/extmk.rb: fix variable name for --extstatic option.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45886 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
extmk.rb: check if parent is build

* ext/extmk.rb (extmake): don't build nested libraries if parent
  library isn't build.

* ext/{dl/callback,tk/tkutil}/extconf.rb: no longer need to check
  if the parent is build.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45887 b2dd03c8-39d4-4d8f-98ff-823fe69b080e