dune

(library
 (name l)
 (modules (:standard \ main))
 (flags (:standard -w -33))
 (libraries
  threads                       ; must be first
  unix
 )
 (preprocess (pps lwt_ppx ppx_deriving.std)))

(executable
 (name main)
 (modules main)
 (libraries l extlib)
 (preprocess (pps lwt_ppx ppx_deriving.std)))

foo.ml

let a = 3

main.ml

let () =
  print_int L.Foo.a