ffado: prevent build tools from leaking into closure

FFADO stored paths to various build dependencies and libraries
like gcc or pyuic5 (from PyQT) in $out/lib/libffado/static_info.txt,
thus  bringing them into the runtime closure.

With Nix, this information is not really critical, as we can find
out the exact dependencies from .drv files in Nix store.

This alone reduced the closure size from 914866184B to 132341176B.

pkgs/os-specific/linux/ffado/default.nix

@@ -53,6 +53,11 @@ in stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
 
+  postInstall = ''
+    # prevent build tools from leaking into closure
+    echo 'See `nix-store --query --tree ${placeholder "out"}`.' > $out/lib/libffado/static_info.txt
+  '';
+
   meta = with stdenv.lib; {
     homepage = http://www.ffado.org;
     description = "FireWire audio drivers";