pkgs: add havm

This is a dependency for Tiger Compiler [1].

[1]: https://assignments.lrde.epita.fr/

pkgs/default.nix

@@ -1,5 +1,7 @@
 { pkgs }:
 {
+  havm = pkgs.callPackage ./havm { };
+
   lohr = pkgs.callPackage ./lohr { };
 
   nolimips = pkgs.callPackage ./nolimips { };

pkgs/havm/default.nix

@@ -0,0 +1,30 @@
+{ fetchurl, ghc, lib, stdenv, which }:
+stdenv.mkDerivation rec {
+  pname = "havm";
+  version = "0.28";
+
+  src = fetchurl {
+    url = "https://www.lrde.epita.fr/~tiger/download/${pname}-${version}.tar.gz";
+    sha256 = "sha256-FDi4FZ8rjGqRkFlROtcJsv+mks7MmIXQGV4bZrwkQrA=";
+  };
+
+  buildInputs = [
+    ghc
+    which # Used by tests
+  ];
+
+  doCheck = true;
+
+  meta = with lib; {
+    description = "A simple virtual machine to execute Andrew Appel's HIR/LIR";
+    longDescription = ''
+      HAVM is a virtual machine designed to execute simple register based high
+      level intermediate code.  It is based on the intermediate representations
+      ("canonicalized" or not) defined by Andrew Appel in his "Modern Compiler
+      Implementation".
+    '';
+    homepage = "https://www.lrde.epita.fr/wiki/Havm";
+    license = licenses.gpl2Plus;
+    platforms = platforms.all;
+  };
+}