fiddle: do not disturb other checks

* ext/fiddle/extconf.rb: add the local ffi library and header just
  before create_makefile, not to disturb other checks.
  also prepend the extension path name to the local library name
  for static linked ext.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48915 b2dd03c8-39d4-4d8f-98ff-823fe69b080e