man-db: clean the expression a bit

This solves the errors
> can't open the manpath configuration file /etc/man_db.conf
which were common e.g. for fish users.

pkgs/tools/misc/man-db/default.nix

@@ -8,12 +8,17 @@ stdenv.mkDerivation rec {
     sha256 = "056a3il7agfazac12yggcg4gf412yq34k065im0cpfxbcw6xskaw";
   };
 
-  buildInputs = [ pkgconfig libpipeline db groff ];
+  outputs = [ "out" "doc" ];
+  outputMan = "out"; # users will want `man man` to work
+
+  nativeBuildInputs = [ pkgconfig ];
+  buildInputs = [ libpipeline db groff ];
 
   configureFlags = [
     "--disable-setuid"
-    "--sysconfdir=/etc"
     "--localstatedir=/var"
+    # Don't try /etc/man_db.conf by default, so we avoid error messages.
+    "--with-config-file=\${out}/etc/man_db.conf"
     "--with-systemdtmpfilesdir=\${out}/lib/tmpfiles.d"
     "--with-eqn=${groff}/bin/eqn"
     "--with-neqn=${groff}/bin/neqn"
@@ -23,15 +28,9 @@ stdenv.mkDerivation rec {
     "--with-tbl=${groff}/bin/tbl"
   ];
 
-  installFlags = [ "DESTDIR=\${out}" ];
+  enableParallelBuilding = true;
 
-  postInstall = ''
-    mv $out/$out/* $out
-    DIR=$out/$out
-    while rmdir $DIR 2>/dev/null; do
-      DIR="$(dirname "$DIR")"
-    done
-  '';
+  doCheck = true;
 
   meta = with stdenv.lib; {
     homepage = "http://man-db.nongnu.org";