extconf.rb: libffi on mswin

* ext/fiddle/extconf.rb: try to compile libffi as mingw on mswin
  if possible.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48925 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
extconf.rb: try modversion ff pkg_config succeeded

* ext/fiddle/extconf.rb: when pkg_config failed, module version is
  not available.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48926 b2dd03c8-39d4-4d8f-98ff-823fe69b080e