Add Configurator.V1.write_flags

See https://github.com/ocaml/dune/pull/696/files#r181721525

doc/configurator.rst

@@ -80,6 +80,11 @@ this flag file using ``:include``:
     (c_names (foo))
     (c_library_flags (:include (flags.sexp)))))
 
+For this, generate the list of flags for your library — for example
+using ``Configurator.V1.Pkg_config`` — and then write them to a file,
+in the above example ``flags.sexp``, with
+``Configurator.V1.write_flags "flags.sexp" flags``.
+
 Upgrading from the old Configurator
 ===================================
 

src/configurator/v1.ml

@@ -419,6 +419,10 @@ module Pkg_config = struct
       None
 end
 
+let write_flags fname s =
+  let sexp = Usexp.List(List.map ~f:Usexp.atom_or_quoted_string s) in
+  Io.write_file fname (Usexp.to_string sexp)
+
 let main ?(args=[]) ~name f =
   let ocamlc  = ref (
     match Sys.getenv "DUNE_CONFIGURATOR" with

src/configurator/v1.mli

@@ -88,6 +88,11 @@ module Pkg_config : sig
   val query : t -> package:string -> package_conf option
 end with type configurator := t
 
+val write_flags : string -> string list -> unit
+(** [write_flags fname s] write the list of strings [s] to the file
+   [fname] in an appropriate format so that it can used in jbuild
+   files with "(:include [fname])". *)
+
 (** Typical entry point for configurator programs *)
 val main
   :  ?args:(Arg.key * Arg.spec * Arg.doc) list