[Buildroot] [git commit branch/2017.08.x] core/reproducible: do not override SOURCE_DATE_EPOCH

[Peter Korsgaard]

commit: https://git.buildroot.net/buildroot/commit/?id=1ecf38cb2fa82ccbfdcb60fd4115e494feda2733
branch: https://git.buildroot.net/buildroot/commit/?id=refs/heads/2017.08.x

SOURCE_DATE_EPOCH is currently forcibly set (to either the git commit
date, or the last release date).

However, the spec mandates that it should not be modified if already
set: https://reproducible-builds.org/specs/source-date-epoch/

    Build systems MUST NOT overwrite this variable for child
    processes to consume if it is already present.

Abide by the rule, and only set it if not already set.

This will allow users to pass it from an upper-layer buildsystem (e.g. a
jenkins or gitlab-ci job, for example), when they have a reson to do so.

Reported-by: Peter Korsgaard <peter at korsgaard.com>
Reported-by: Einar Jón Gunnarsson <tolvupostur at gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998 at free.fr>
Cc: Peter Korsgaard <peter at korsgaard.com>
Cc: Einar Jón Gunnarsson <tolvupostur at gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
Cc: Arnout Vandecappelle <arnout at mind.be>
Signed-off-by: Peter Korsgaard <peter at korsgaard.com>
(cherry picked from commit 0437d2f8f6ef8c10b529c154f4534453ce34607b)
Signed-off-by: Peter Korsgaard <peter at korsgaard.com>
---
 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index eb02092..39db0c1 100644
--- a/Makefile
+++ b/Makefile
@@ -254,7 +254,7 @@ export LANG = C
 export LC_ALL = C
 export GZIP = -n
 BR2_VERSION_GIT_EPOCH = $(shell GIT_DIR=$(TOPDIR)/.git $(GIT) log -1 --format=%at)
-export SOURCE_DATE_EPOCH = $(if $(wildcard $(TOPDIR)/.git),$(BR2_VERSION_GIT_EPOCH),$(BR2_VERSION_EPOCH))
+export SOURCE_DATE_EPOCH ?= $(if $(wildcard $(TOPDIR)/.git),$(BR2_VERSION_GIT_EPOCH),$(BR2_VERSION_EPOCH))
 DEPENDENCIES_HOST_PREREQ += host-fakedate
 endif