Support for Ruby 2.0 >= p195

It looks like CFLAGS stopped being added to the CXXFLAGS.
This should still be fine for 1.9 and 1.8 (as far as my testing can tell), but I can make this change 2.0 specific if you'd like.

Commit in ruby: https://github.com/ruby/ruby/commit/2e26edb633606f1e99d22817678e11c25f7d2882