Since last time I've checked mruby, many things had changed. The biggest one was the introduction of compiled-time plugins that were confusingly called mrbgems. I have a completely different image in mind when I hear words ruby & gem together.
Still no love for require from matz.
To get an interpreter that is useful IRL, it's possible to cherry pick from a list of mrbgems. mruby-require plugin, sorry, gem is the most confusing one. If you specify it before other plugins, sorry, gems, all other gems (below it) will be compiled as .so libs & to use them you would write require foo & would immediately lose compatibility with MRI. After that, the helper
def mruby? RUBY_ENGINE == 'mruby' end
& conditional checks is the only answer.
mruby build system is interesting. It uses a nano-version of Rake called minirake. By an unknown reason it's incompatible with mruby. At that point I thought "How would be cool to have rake as a standalone executable that doesn't depend on Ruby at all?".
What it has to do with mruby? It turns out, mruby can produce an array of bytecode that can be compiled with your C program into 1 executable.
It sounds cool but has its limitations. Firstly, you'll need to inline all your require statements to have 1 .rb source file. Secondly, remember, there is no stdlib in mruby. Plugins, sorry, gems, that try to bring it to mruby are nice but incomplete (for example, Dir misses glob).
You'll find problems in areas you've never imagined. For example, Ruby ISO standard doesn't mention ARGV & $0 (that't what I heard, the pdf paper is under 198 CHF paywall) which means, right, no ARGV & $0 by default--you'll need to look in mirb src to guess how to inject them.
Btw, googling won't help much, because most blog posts about mruby were written in 2012 & API is different now => old examples are mostly useless.
Back to rake. Porting 'real' rake is a daunting task. I just took minirake source, tweaked them a bit & wrote a tiny C wrapper with a couple of rakefiles: https://github.com/gromnitsky/minirake. Amazingly it seems to work. Glory to Japan!