Allow to override build date with SOURCE_DATE_EPOCH

in order to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

While it is already possible to do reproducible builds without
this patch, this patch makes it the default,
so that distributions do not have to discover the BUILD_INFO variable.

Signed-off-by: Bernhard M. Wiedemann <bwiedemann@suse.de>

Makefile

@@ -35,7 +35,11 @@ OCIUMOUNTINSTALLDIR=$(PREFIX)/share/oci-umount/oci-umount.d
 
 SELINUXOPT ?= $(shell selinuxenabled 2>/dev/null && echo -Z)
 
-BUILD_INFO := $(shell date +%s)
+ifdef SOURCE_DATE_EPOCH
+    BUILD_INFO := ${SOURCE_DATE_EPOCH}
+else
+    BUILD_INFO := $(shell date +%s)
+endif
 
 GO_MD2MAN := ${BUILD_BIN_PATH}/go-md2man
 GINKGO := ${BUILD_BIN_PATH}/ginkgo