[question] Using custom threading library

I'm using RocksDB in a program that relies on a custom threading library. It's works fine because RocksDB spawns and manage its own POSIX threads, and because I make sure I access RocksDB from only one of my custom threads. Now, I could get more performance if I could make RocksDB use my threads and mutex instead of its own. I have seen various abstract classes like `ThreadPool` in the RocksDB headers. Is it possible to inject a custom threading layer into RocksDB via some of its abstract classes? Essentially I would need to make RocksDB not just use my threads, but also custom mutex/lock/condition variable classes that my threading library uses, in place of the POSIX ones.
How would you go about doing that?