guix: add parameters for certain configure flags

This is in preparation for the Nix module where it will allow the user
to set custom store and state directory.

pkgs/by-name/gu/guix/package.nix

···
 , bzip2
 , libgcrypt
 , sqlite
+
+, stateDir ? "/var"
+, storeDir ? "/gnu/store"
+, confDir ? "/etc"
 }:
 
 stdenv.mkDerivation rec {
···
   ];
 
   configureFlags = [
-    "--localstatedir=/var"
-    "--sysconfdir=/etc"
+    "--with-store-dir=${storeDir}"
+    "--localstatedir=${stateDir}"
+    "--sysconfdir=${confDir}"
     "--with-bash-completion-dir=$(out)/etc/bash_completion.d"
   ];