For understandable reasons, Crystal doesn't ship linking files out with the Crystal repo, since they are platform specific. However, because of that the Crystalline build fails unless an extra manual step is taken, and this manual step is not easily discovered without notable coding expertise or significant searching.
I think it would be appropriate for Crystalline to detect the existence of llvm_ext.o and build it as part of the Crystalline build process if it is missing to alleviate this.