fiddle: configure in mflags

* ext/fiddle/extconf.rb: configure is already in mflags, use
  system instead of xsystem which requires a string, and logging
  libffi configure output.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48904 b2dd03c8-39d4-4d8f-98ff-823fe69b080e