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.

Without this patch, openSUSE's qemu package
always varied between builds.

This patch was done while working on reproducible builds for openSUSE.

This is an alternative to
https://lists.ozlabs.org/pipermail/slof/2023-October/002895.html

Signed-off-by: Bernhard M. Wiedemann <bwiedemann@suse.de>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>

romfs/tools/create_crc.c

diff --git a/romfs/tools/create_crc.c b/romfs/tools/create_crc.c
index abc373b..4a30e48 100644
--- a/romfs/tools/create_crc.c
+++ b/romfs/tools/create_crc.c
@@ -95,7 +95,13 @@
 
 	if (!notime) {
 		/* read time and write it into data stream */
-		if ((caltime = time(NULL)) == -1) {
+		char *source_date_epoch;
+		/* This assumes that (if set) the SOURCE_DATE_EPOCH environment variable
+		   will contain a correct, positive integer in the time_t range */
+		if ((source_date_epoch = getenv("SOURCE_DATE_EPOCH")) == NULL ||
+		    (caltime = (time_t)strtoll(source_date_epoch, NULL, 10)) <= 0)
+			caltime = time(NULL);
+		if (caltime == -1) {
 			printf("time error\n");
 		}
 		if ((tm = localtime(&caltime)) == NULL) {