kdePackages.elisa: Allow building without libVLC

Elisa supports using qtmultimedia's own facilities for audio playback,
rather than using Elisa's own custom VLC-based backend, which it will
prefer if available at build time. qtmultimedia's default backend on
most platforms is ffmpeg, and anecdotally, it's giving me better
performance than Elisa's VLC-based backend was.

It seems like most distributions build it with libVLC, with the notable
exceptions of Fedora, and Gentoo (which of course makes building with
VLC user-configurable).

pkgs/kde/gear/elisa/default.nix

@@ -1,14 +1,14 @@
 {
+  lib,
   mkKdeDerivation,
   qtmultimedia,
   libvlc,
+
+  withVLC ? true,
 }:
 mkKdeDerivation {
   pname = "elisa";
 
-  extraBuildInputs = [
-    qtmultimedia
-    libvlc
-  ];
+  extraBuildInputs = [ qtmultimedia ] ++ lib.optional withVLC libvlc;
   meta.mainProgram = "elisa";
 }