While I have no experience building ruby, if standard ruby gives you trouble - might be worth to try mruby - it is likely easier to build and debug the build process.
Counterintuitively, having messed around with building mruby for a project a few months ago, I found it harder to deal with. It's a lot more hardcore, with vastly fewer resources and more sharp edges, and is orders of magnitude less popular, so you find yourself hitting basic problems you would think would have been solved years ago.
I'd stay away from it unless you have a very pressing need.