Ruby 2.0 compatibility (includes 1.9.3 fixes)

This fixes rqr on ruby 2.0:
* Use fiddle instead of dl if available (fixes "DL is deprecated, please use Fiddle" warning)
* Fix extconf.rb to properly set macros in the proper environment var (fix missing jpeg/png/tiff support on ruby 2.0)

It's based on the `ruby_1_9_3` branch, so it also includes those fixes.

I've bumped the version to 0.2.6 in my fork's master, but left it out of the PR. Don't forget to bump if you merge this.