From: Hannes Mehnert <hannes@mehnert.org>
Subject: [PATCH 1/1] compile mirage-entropy sublibrary if mirage-entropy is installed

Previously the compilation depended on mirage-xen or ocaml-freestanding being
present, but _tags show the dependency is actually mirage-entropy.

--- a/opam	2019-11-02 13:38:17.202429000 +0100
+++ b/opam	2019-11-02 13:39:19.173535000 +0100
@@ -12,6 +12,7 @@
 
 build: ["ocaml" "pkg/pkg.ml" "build" "--pinned" "%{pinned}%" "--tests" "false"
                 "--with-lwt" "%{lwt:installed}%"
+                "--with-mirage" "%{mirage-entropy:installed}%"
                 "--xen" "%{mirage-xen:installed}%"
                 "--freestanding" "%{ocaml-freestanding:installed}%"]
 
@@ -26,11 +27,11 @@
   "cstruct"
   "zarith"
   "sexplib"
-  ("mirage-no-xen" | ("mirage-xen" & "mirage-entropy" & "zarith-xen"))
-  ("mirage-no-solo5" | ("mirage-solo5" & "mirage-entropy" & "zarith-freestanding"))
+  ("mirage-no-xen" | ("mirage-xen" & "zarith-xen"))
+  ("mirage-no-solo5" | ("mirage-solo5" & "zarith-freestanding"))
 ]
 
-depopts: [ "lwt" ]
+depopts: [ "lwt" "mirage-entropy" ]
 conflicts: [
   "topkg" {<"0.8.0"}
   "ocb-stubblr" {<"0.1.0"}
--- a/pkg/pkg.ml	2017-02-01 00:43:04.000000000 +0100
+++ b/pkg/pkg.ml	2019-11-02 13:37:30.295780000 +0100
@@ -11,6 +11,7 @@
 
 let unix = Conf.with_pkg ~default:true "unix"
 let lwt  = Conf.with_pkg ~default:false "lwt"
+let mirage_entropy = Conf.with_pkg ~default:false "mirage"
 let xen  = Conf.(key "xen" bool ~absent:false
                  ~doc:"Build Mirage/Xen support.")
 let fs   = Conf.(key "freestanding" bool ~absent:false
@@ -39,12 +40,14 @@
     let unix = Conf.value c unix in
     let lwt  = Conf.value c lwt && unix
     and xen  = Conf.value c xen
-    and fs   = Conf.value c fs in
+    and fs   = Conf.value c fs
+    and mirage_entropy = Conf.value c mirage_entropy
+    in
     Ok [ Pkg.clib "src/libnocrypto_stubs.clib";
          Pkg.mllib "src/nocrypto.mllib";
          Pkg.mllib ~cond:unix "unix/nocrypto_unix.mllib";
          Pkg.mllib ~cond:lwt "lwt/nocrypto_lwt.mllib";
-         Pkg.mllib ~cond:(xen||fs) "mirage/nocrypto_mirage.mllib";
+         Pkg.mllib ~cond:mirage_entropy "mirage/nocrypto_mirage.mllib";
          Pkg.test "tests/testrunner";
          Pkg.test ~run:false "bench/speed";
          mirage ~xen ~fs "src/libnocrypto_stubs.clib"; ]