add an ocaml specific builder to the system

pkgs/build-support/ocaml/default.nix

···
+{ stdenv, writeText, ocaml, findlib, camlp4 }:
+
+{ name, version, buildInputs ? [],
+  createFindlibDestdir ?  true,
+  dontStrip ? true,
+  minimumSupportedOcamlVersion ? null,
+  hasSharedObjects ? false,
+  setupHook ? null,
+  meta ? {}, ...
+}@args:
+let
+  ocaml_version = (builtins.parseDrvName ocaml.name).version;
+  defaultMeta = {
+    platforms = ocaml.meta.platforms;
+  };
+in
+  assert minimumSupportedOcamlVersion != null ->
+          stdenv.lib.versionOlder minimumSupportedOcamlVersion ocaml_version;
+
+stdenv.mkDerivation (args // {
+  name = "ocaml-${name}-${version}";
+
+  buildInputs = [ ocaml findlib camlp4 ] ++ buildInputs;
+
+  setupHook = if setupHook == null && hasSharedObjects
+  then writeText "setupHook.sh" ''
+    export CAML_LD_LIBRARY_PATH="''${CAML_LD_LIBRARY_PATH}''${CAML_LD_LIBRARY_PATH:+:}''$1/lib/ocaml/${ocaml_version}/site-lib/${name}/"
+    ''
+  else setupHook;
+
+  inherit ocaml_version;
+  inherit createFindlibDestdir;
+  inherit dontStrip;
+
+  meta = defaultMeta // meta;
+})

pkgs/top-level/all-packages.nix

···
       ocaml_version = (builtins.parseDrvName ocaml.name).version;
     in rec {
     inherit ocaml;
+    buildOcaml = callPackage ../build-support/ocaml { };
 
     acgtk = callPackage ../applications/science/logic/acgtk { };