wireguard: Enable tools on other platforms

Wireguard is now split into two pretty much independent packages:
`wireguard` (Linux-specific kernel module) and `wireguard-tools`,
which is cross-platform.

pkgs/tools/networking/wireguard-tools/default.nix

@@ -0,0 +1,46 @@
+{ stdenv, lib, fetchurl, libmnl, useSystemd ? stdenv.isLinux }:
+
+let
+  inherit (lib) optional optionalString;
+in
+
+stdenv.mkDerivation rec {
+  name = "wireguard-tools-${version}";
+  version = "0.0.20180514";
+
+  src = fetchurl {
+    url    = "https://git.zx2c4.com/WireGuard/snapshot/WireGuard-${version}.tar.xz";
+    sha256 = "1nk6yj1gdmpar99zzw39n1v795m6fxsrilg37d02jm780rgbd5g8";
+  };
+
+  preConfigure = "cd src";
+
+  buildInputs = optional stdenv.isLinux libmnl;
+
+  enableParallelBuilding = true;
+
+  makeFlags = [
+    "WITH_BASHCOMPLETION=yes"
+    "WITH_WGQUICK=yes"
+    "WITH_SYSTEMDUNITS=${if useSystemd then "yes" else "no"}"
+    "DESTDIR=$(out)"
+    "PREFIX=/"
+    "-C" "tools"
+  ];
+
+  buildPhase = "make tools";
+
+  postInstall = optionalString useSystemd ''
+    substituteInPlace $out/lib/systemd/system/wg-quick@.service \
+      --replace /usr/bin $out/bin
+  '';
+
+  meta = with stdenv.lib; {
+    homepage     = https://www.wireguard.com/;
+    downloadPage = https://git.zx2c4.com/WireGuard/refs/;
+    description  = " Tools for the WireGuard secure network tunnel";
+    maintainers  = with maintainers; [ ericsagnes mic92 zx2c4 ];
+    license      = licenses.gpl2;
+    platforms    = platforms.unix;
+  };
+}