fiddle: fix build failures

* ext/fiddle/depend (build-libffi): get rid of eventually empty
  target for nmake.

* ext/fiddle/extconf.rb: omit stuffs for libffi unless bundled
  libffi is found and used.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48913 b2dd03c8-39d4-4d8f-98ff-823fe69b080e