Allow custom compilation flags for ruby installation

[ben-ballot]

Hello,

Following some discussion with jemalloc and ruby (https://bugs.ruby-lang.org/issues/14718),
it would be great to have a way to render the option in the compilation phase so jemalloc could be used.

Regards,

[adam-stokes]

I'm totally opened to this, I've got a lot on my plate right now so if you (or anyone else) wants to take a stab at it I'd happily review