Override mtime with zip -X

with SOURCE_DATE_EPOCH
to allow for reproducible builds of .zip files

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.

Uses clamping to keep older mtimes than SOURCE_DATE_EPOCH intact.

zipup.c

@@ -414,6 +414,7 @@ struct zlist far *z;    /* zip entry to compress */
   ush tempcext = 0;
   char *tempextra = NULL;
   char *tempcextra = NULL;
+  const char *source_date_epoch;
 
 
 #ifdef WINDLL
@@ -674,6 +675,13 @@ struct zlist far *z;    /* zip entry to compress */
 
   } /* strcmp(z->name, "-") == 0 */
 
+  if (extra_fields == 0 && (source_date_epoch = getenv("SOURCE_DATE_EPOCH")) != NULL) {
+     time_t epoch = strtoull(source_date_epoch, NULL, 10);
+     if (epoch > 0) {
+       ulg epochtim = unix2dostime(&epoch);
+       if (z->tim > epochtim) z->tim = epochtim;
+     }
+  }
   if (extra_fields == 2) {
     unsigned len;
     char *p;