F star compiles to Ocaml or F#. Here's the page on integrating Ocaml and C:

http://caml.inria.fr/pub/docs/manual-ocaml/intfc.html

I'm not sure at a glance how easy it will be to use F star programs in C applications via Ocaml. We'd be better off if there was a compiler from F star to annotated/safe C, Ada/SPARK, or Rust. All of these can integrate more easily into the legacy apps.