Fix extconf to use absolute paths

[cheald]

I needed to use rb-gsl in one of my gems, but the version on rubygems doesn't build. So, I'm vendoring this in my gem's ext/ dir, and having rb-gsl build on gem install via the gemspec directives. In order for this to work, the paths extconf uses need to be absolute rather than relative; this patch simply fixes the paths so that the gem can be built as a sub-extension to another gem.