dune

(rule
 (target dune-linker)
 (enabled_if %{bin-available:lld})
 (action
  (with-stdout-to
   dune-linker
   (echo "lld"))))

(rule
 (target dune-linker)
 (enabled_if
  (= %{bin-available:lld} false))
 (action
  (with-stdout-to
   dune-linker
   (echo "bfd"))))

(env
 (dev
  (flags
   (:standard -ccopt=-fuse-ld=%{read:dune-linker})))
 (dev
  (ocamlopt_flags
   (:standard -O3))))