In that case, the system ruby is used to re-check encoding sources and update encdb.h if needed.
Encodings under enc directory will be added to encdb.h automatically by template/encdb.h.tmpl file.
We assume that the cost to re-check using ruby is much less than the cost for rebuild all encodings, or the cost to re-check using other than ruby.
Another assumption is that the system ruby provides full of bundled libraries, ArchLinux is not the case apparently.
I'll add a check that the system ruby matches our assumption.
This issue happens only when ruby is installled but erb is not installed. 6ab591f80aa19d63ecd1e1df3c09c391efb318a6 checks if not only ruby but also erb is installed. So I think the issue itself is already resolved. Using miniruby for rebuilding encdb.h is a different issue.