FIL-plugins: fix cross compilation

pkgs/applications/audio/FIL-plugins/default.nix

···
-{ lib, stdenv, fetchurl, ladspaH
+{ lib
+, stdenv
+, fetchurl
+, ladspaH
 }:
 
 stdenv.mkDerivation rec {
   pname = "FIL-plugins";
   version = "0.3.0";
+
   src = fetchurl {
     url = "http://kokkinizita.linuxaudio.org/linuxaudio/downloads/${pname}-${version}.tar.bz2";
-    sha256 = "1scfv9j7jrp50r565haa4rvxn1vk2ss86xssl5qgcr8r45qz42qw";
+    hash = "sha256-HAvycSEZZfZwoVp3g7QWcwfbdyZKwWJKBuVmeWTajuk=";
   };
 
   buildInputs = [ ladspaH ];
 
-  patchPhase = ''
-    sed -i 's@/usr/bin/install@install@g' Makefile
-    sed -i 's@/bin/rm@rm@g' Makefile
-    sed -i 's@/usr/lib/ladspa@$(out)/lib/ladspa@g' Makefile
+  postPatch = ''
+    substituteInPlace Makefile \
+      --replace /usr/lib/ladspa "$out/lib/ladspa" \
+      --replace g++             "$CXX"
   '';
 
-  preInstall="mkdir -p $out/lib/ladspa";
+  preInstall = ''
+    mkdir -p "$out/lib/ladspa"
+  '';
 
   meta = {
     description = "a four-band parametric equaliser, which has the nice property of being stable even while parameters are being changed";