Add support for setting build command when not set

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/opam_create.ml

@@ -1,5 +1,17 @@
 open Stdune
 
+let default_build_command = lazy (
+  let path = Path.of_string "<internal>" in
+  let dummy_opam =
+      Opam_file.of_string ~path {|
+build:
+  [ "dune" "subst" ] {pinned}
+  [ "dune" "build" "-p" name "-j" jobs]
+|}
+  in
+  Opam_file.get_field dummy_opam "build"
+  |> Option.value_exn)
+
 let correct_specific
       { Package.synopsis
       ; description
@@ -27,6 +39,7 @@ let correct project (package : Package.t) =
   let open Opam_file.Mutator in
   let open Opam_file.Mutator.No_overwrite in
   correct_specific package >>>
+  set_var "build" (Lazy.force default_build_command) >>>
   opt (Dune_project.maintainer project) (set_string "maintainer") >>>
   opt (Dune_project.bug_reports project) (set_string "bug-reports") >>>
   opt (Dune_project.homepage project) (set_string "homepage") >>>

src/opam_file.ml

@@ -139,7 +139,7 @@ module Mutator = struct
   module Make_set(Set_var : sig
       val set_var : string -> value -> t
     end) = struct
-    open Set_var
+    include Set_var
     let set_string x y = set_var x (mkstring y)
     let set_list x conv l = set_var x (mklist conv l)
   end