Add support for printing qr code for google-authenticator.

pkgs/os-specific/linux/google-authenticator/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, pam }:
+{ stdenv, fetchurl, pam, qrencode }:
 
 stdenv.mkDerivation rec {
   name = "google-authenticator-1.0";
@@ -10,6 +10,10 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ pam ];
 
+  preConfigure = ''
+    sed -i 's|libqrencode.so.3|${qrencode}/lib/libqrencode.so.3|' google-authenticator.c
+  '';
+
   installPhase = ''
     ensureDir $out/bin $out/lib/security
     cp pam_google_authenticator.so $out/lib/security