libguestfs: fix build with OCaml 4.12

pkgs/development/libraries/libguestfs/default.nix

@@ -46,7 +46,7 @@ stdenv.mkDerivation rec {
   '';
   configureFlags = [ "--disable-appliance" "--disable-daemon" "--with-distro=NixOS" ]
     ++ lib.optionals (!javaSupport) [ "--disable-java" "--without-java" ];
-  patches = [ ./libguestfs-syms.patch ];
+  patches = [ ./libguestfs-syms.patch ./ocaml-4.12.patch ];
   NIX_CFLAGS_COMPILE="-I${libxml2.dev}/include/libxml2/";
   installFlags = [ "REALLY_INSTALL=yes" ];
   enableParallelBuilding = true;

pkgs/development/libraries/libguestfs/ocaml-4.12.patch

@@ -0,0 +1,11 @@
+--- a/common/mlstdutils/std_utils.ml	2019-02-07 15:45:56.516955598 +0100
++++ b/common/mlstdutils/std_utils.ml	2019-02-07 15:45:56.516955598 +0100
+@@ -305,7 +305,7 @@
+       | x::xs, y::ys, z::zs -> (x, y, z) :: combine3 xs ys zs
+       | _ -> invalid_arg "combine3"
+ 
+-    let rec assoc_lbl ?(cmp = compare) ~default x = function
++    let rec assoc_lbl ?(cmp = Pervasives.compare) ~default x = function
+       | [] -> default
+       | (y, y') :: _ when cmp x y = 0 -> y'
+       | _ :: ys -> assoc_lbl ~cmp ~default x ys