What does the final c in bin/jruby-cext-c means? compile? Should it be cc so we relate by name to the standard C compiler? Maybe it could also be jruby-cext-make, as standard workflow to compile a C-ext is: cd ext/dir; ruby extconf.rb; make;.

Yes it means JRuby's C extension compiler. I didn't call it 'cc' as it'll need to be able to compile C++ as well. Do people use other languages as well? make might be a better idea. Hopefully this isn't for the long term though, as we should modify the rake plugin at some point.

I know of extensions written in Rust used in the wild. I've seen interesting in writing them in Go, which is evidently possible now.