Add support for >= Ruby 2.0.0-p195

For some reason Ruby 2.0 versions with a patchlevel > 0 have trouble building google_hash.
It looks like something called CCDLFLAGS was introduced and CFLAGS stopped being added to the CXXFLAGS.

Commit in ruby: https://github.com/ruby/ruby/commit/2e26edb633606f1e99d22817678e11c25f7d2882