Use deterministic SOURCE_DATE_EPOCH for embedded timestamps in generated documentation.

Written by Eduard Sanou.

https://bugzilla.gnome.org/show_bug.cgi?id=758148

--- libxslt-1.1.28.orig/libexslt/date.c
+++ libxslt-1.1.28/libexslt/date.c
@@ -46,6 +46,7 @@
 #include "exslt.h"
 
 #include <string.h>
+#include <errno.h>
 
 #ifdef HAVE_MATH_H
 #include <math.h>
@@ -747,21 +748,46 @@ static exsltDateValPtr
 exsltDateCurrent (void)
 {
     struct tm localTm, gmTm;
+    struct tm *tb = NULL;
     time_t secs;
     int local_s, gm_s;
     exsltDateValPtr ret;
+    char *source_date_epoch;
 
     ret = exsltDateCreateDate(XS_DATETIME);
     if (ret == NULL)
         return NULL;
 
-    /* get current time */
     secs    = time(NULL);
+    /*
+     * Allow the date and time to be set externally by an exported
+     * environment variable to enable reproducible builds.
+     */
+    source_date_epoch = getenv("SOURCE_DATE_EPOCH");
+    if (source_date_epoch) {
+	errno = 0;
+	secs = (time_t) strtol (source_date_epoch, NULL, 10);
+	if (errno == 0) {
+	    tb = gmtime(&secs);
+	    if (tb == NULL) {
+	    /* SOURCE_DATE_EPOCH is not a valid date */
+		return NULL;
+	    } else {
+		localTm = *tb;
+	    }
+	} else {
+	    /* SOURCE_DATE_EPOCH is not a valid number */
+	    return NULL;
+	}
+    } else {
+	/* get current time */
 #if HAVE_LOCALTIME_R
-    localtime_r(&secs, &localTm);
+	localtime_r(&secs, &localTm);
 #else
-    localTm = *localtime(&secs);
+	localTm = *localtime(&secs);
 #endif
+    }
+
 
     /* get real year, not years since 1900 */
     ret->value.date.year = localTm.tm_year + 1900;