So, I managed to build https://github.com/HeapsIO/bullet on Linux, both with the sources from https://github.com/bulletphysics/bullet3 (using CMakeFiles.txt) and with libbullet-dev package that comes from Debian/Ubuntu repos (using Makefile) and I got the HeapsIO/bullet/sample/Main.hx to compile and run with HashLink.
At the first glance, I like HeapsIO/bullet and I think I’ll be able to use it for handful of simple demos and/or simulations. I’m wondering about using it for more involved projects though, the main reason being that the last commit to HeapsIO/bullet repo was ~3 years ago.
My question is: what was the reason (or reasons) for not working on it since? Did the Bullet Physics library turned out to be not a good choice for HeapsIO/HashLink combination? Too complex for the use case (games)? Too resource hungry? Not a good choice for games?
If the reason is some kind of incompatibility between how Bullet Physics SDK works and HeapsIO/HashLink, it would be good to know now, before I spend a ton of time on it and discover the same (potential) issues. 
Thanks!