extconf.rb: configure libffi first

* ext/fiddle/extconf.rb: configure libffi for the check requires
  the headers.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48917 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-12-22

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48918 b2dd03c8-39d4-4d8f-98ff-823fe69b080e