Add ctypes-foreign. (ocaml-cross#312)

* Add ctypes-foreign.

* Patch

packages/ctypes-foreign-windows/ctypes-foreign-windows.0.22.0/files/patches/fix-config.patch

@@ -0,0 +1,16 @@
+diff --git a/src/ctypes-foreign/config/discover.ml b/src/ctypes-foreign/config/discover.ml
+index 7be7ae3..85de869 100644
+--- a/src/ctypes-foreign/config/discover.ml
++++ b/src/ctypes-foreign/config/discover.ml
+@@ -14,10 +14,7 @@ let () =
+            | None -> default
+            | Some v -> v)
+       in
+-      let backend =
+-        match Sys.os_type with
+-        | "Win32" | "Cygwin" -> "win"
+-        | _ -> "unix" in
++      let backend = "win" in
+
+       let f = "as_needed_test" in
+       let ml = f ^ ".ml" in

packages/ctypes-foreign-windows/ctypes-foreign-windows.0.22.0/opam

@@ -0,0 +1,47 @@
+opam-version: "2.0"
+synopsis: "Dynamic access to foreign C libraries using Ctypes"
+description: """
+
+This installs the `ctypes-foreign` interface which
+uses `libffi` to provide dynamic access to foreign libraries."""
+maintainer: ["Jeremy Yallop <yallop@gmail.com>"]
+authors: ["Jeremy Yallop"]
+license: "MIT"
+tags: ["org:mirage"]
+homepage: "https://github.com/yallop/ocaml-ctypes"
+doc: "https://ocamllabs.github.io/ocaml-ctypes/"
+bug-reports: "https://github.com/yallop/ocaml-ctypes/issues"
+depends: [
+  "dune" {>= "2.9"}
+  "ocaml-windows" {>= "4.03.0"}
+  "ctypes-windows" {= version}
+  "dune-configurator"
+]
+build: [
+  ["dune" "subst"] {dev}
+  [
+    "dune"
+    "build"
+    "-p"
+    "ctypes-foreign"
+    "-x"
+    "windows"
+    "-j"
+    jobs
+    "@install"
+  ]
+]
+extra-files: [
+  ["patches/fix-config.patch" "md5=aabfea16327b5549e9a559f3ae99ba21"]
+]
+patches: [
+  "patches/fix-config.patch"
+]
+depexts: [
+  ["libffi"] {os-distribution = "mxe"}
+]
+dev-repo: "git+https://github.com/yallop/ocaml-ctypes.git"
+url {
+  src: "https://github.com/yallop/ocaml-ctypes/archive/refs/tags/0.22.0.tar.gz"
+  checksum: "md5=8a301a3e3b79156448a6659859ad506c"
+}

packages/srt-windows/srt-windows.0.3.0/opam

@@ -16,7 +16,7 @@ depends: [
   "conf-pkg-config" {build}
   "dune" {> "2.0"}
   "dune-configurator" {build}
-  "ctypes-windows"
+  "ctypes-foreign-windows"
   "integers-windows"
   "posix-socket-windows"
   "posix-socket"