ocamlPackages.buildOasis: fix handling of installation prefix

pkgs/build-support/ocaml/oasis.nix

@@ -23,7 +23,7 @@ stdenv.mkDerivation (args // {
   buildPhase = ''
     runHook preBuild
     oasis setup
-    ocaml setup.ml -configure
+    ocaml setup.ml -configure --prefix $OCAMLFIND_DESTDIR --exec-prefix $out
     ocaml setup.ml -build
     runHook postBuild
   '';
@@ -37,9 +37,7 @@ stdenv.mkDerivation (args // {
   installPhase = ''
     runHook preInstall
     mkdir -p $out
-    sed -i s+/usr/local+$out+g setup.ml
-    sed -i s+/usr/local+$out+g setup.data
-    prefix=$OCAMLFIND_DESTDIR ocaml setup.ml -install
+    ocaml setup.ml -install
     runHook postInstall
   '';