ruby-build has a [cross-platform md5 wrapper](https://github.com/sstephenson/ruby-build/blob/master/bin/ruby-build#L148-L160). To test whether it's supported, use [`[ -n "$HAS_MD5_SUPPORT" ]`.](https://github.com/sstephenson/ruby-build/blob/master/bin/ruby-build#L686-L691)