xbill: fix build on darwin; test that binary exists (#435464)

pkgs/by-name/xb/xbill/package.nix

@@ -62,10 +62,17 @@ stdenv.mkDerivation rec {
     })
   ];
 
+  makeFlags = "-B";
+
   postInstall = ''
     install -Dm644 pixmaps/icon.xpm $out/share/pixmaps/xbill.xpm
   '';
 
+  doInstallCheck = true;
+  postInstallCheck = ''
+    $out/bin/xbill --version
+  '';
+
   meta = {
     description = "Protect a computer network from getting infected";
     homepage = "http://www.xbill.org/";