bug#25598: R packages are not bit-reproducible

[Ricardo Wurmus]

It looks like R 3.3.2 already includes the fixes but they need to be
explicitly requested when installing packages.

Attached is a patch that seems to fix this.

>From fa42971cb7099e3b370565de5d3f454faecf0369 Mon Sep 17 00:00:00 2001
From: Ricardo Wurmus <address@hidden>
Date: Wed, 1 Feb 2017 11:42:34 +0100
Subject: [PATCH] build: r-build-system: Use deterministic built date.

Fixes <http://bugs.gnu.org/25598>.

* guix/build/r-build-system.scm (install): Pass "--built-timestamp"
option to make build deterministic.
---
 guix/build/r-build-system.scm | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/guix/build/r-build-system.scm b/guix/build/r-build-system.scm
index 3fc13eb83..24aa73d4f 100644
--- a/guix/build/r-build-system.scm
+++ b/guix/build/r-build-system.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2015 Ricardo Wurmus <address@hidden>
+;;; Copyright © 2015, 2017 Ricardo Wurmus <address@hidden>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -84,6 +84,7 @@
          (params       (append configure-flags
                                (list "--install-tests"
                                      (string-append "--library=" site-library)
+                                     "--built-timestamp=1970-01-01"
                                      ".")))
          (site-path    (string-append site-library ":"
                                       (generate-site-path inputs))))
-- 
2.11.0