ocaml, unison: make them build on mips (native compilers)

I switch off the build of ocaml compilers to native code, and add
a 'passthru' that unison can use to see if it needs to call the native
or the bytecode compiler.

pkgs/development/compilers/ocaml/3.12.1.nix

@@ -1,8 +1,8 @@
 { stdenv, fetchurl, ncurses, x11 }:
 
 let
-   useX11 = !stdenv.isArm;
-   useNativeCompilers = !stdenv.isArm;
+   useX11 = !stdenv.isArm && !stdenv.isMips;
+   useNativeCompilers = !stdenv.isArm && !stdenv.isMips;
    inherit (stdenv.lib) optionals optionalString;
 in
 
@@ -30,6 +30,10 @@ stdenv.mkDerivation rec {
     ln -sv $out/lib/ocaml/caml $out/include/caml
   '';
 
+  passthru = {
+    nativeCompilers = useNativeCompilers;
+  };
+
   meta = {
     homepage = http://caml.inria.fr/ocaml;
     licenses = [ "QPL" /* compiler */ "LGPLv2" /* library */ ];