b2sum: use fetchzip, clean up

pkgs/tools/security/b2sum/default.nix

···
-{ stdenv, fetchurl, openmp ? null }:
+{ stdenv, fetchzip, openmp ? null }:
+
+with stdenv.lib;
 
 stdenv.mkDerivation rec {
-  version = "unstable-2018-06-11";
-  rev = "320c325437539ae91091ce62efec1913cd8093c2";
   name = "b2sum-${version}";
+  version = "unstable-2018-06-11";
 
-  src = fetchurl {
-    url = "https://github.com/BLAKE2/BLAKE2/archive/${rev}.tar.gz";
-    sha256 = "19f07dwli9ymlc87ikn84j4h5fv57afwj9ni7s0jkaym5l0q6nqw";
+  src = fetchzip {
+    url = "https://github.com/BLAKE2/BLAKE2/archive/320c325437539ae91091ce62efec1913cd8093c2.tar.gz";
+    sha256 = "0agmc515avdpr64bsgv87wby2idm0d3wbndxzkhdfjgzhgv0rb8k";
   };
-  postUnpack = "sourceRoot=$sourceRoot/b2sum";
+
+  sourceRoot = "source/b2sum";
 
   buildInputs = [ openmp ];
 
-  makeFlags = stdenv.lib.optional (isNull openmp) "NO_OPENMP=1";
+  buildFlags = [ (optional (isNull openmp) "NO_OPENMP=1") ];
   installFlags = [ "PREFIX=$(out)" ];
 
-  meta = with stdenv.lib; {
+  meta = {
     description = "The b2sum utility is similar to the md5sum or shasum utilities but for BLAKE2";
     homepage = "https://blake2.net";
     license = with licenses; [ asl20 cc0 openssl ];