Use SOURCE_DATE_EPOCH to build man pages.

Expand c02f755 to use the
SOURCE_DATE_EPOCH environment variable to create the timestamps
in the .nro man page files.
Also change mkhelp.py to use SOURCE_DATE_EPOCH like mkhelp.pl.

Related to #583.

Makefile.aut

@@ -90,7 +90,8 @@ distclean: clean
 realclean: clean
 
 REPLACE_VERSION = \
-	DT=`date '+%d %h %Y'`; \
+	if [ -n "${SOURCE_DATE_EPOCH}" ]; then DFLAG="-d@${SOURCE_DATE_EPOCH}"; fi; \
+	DT=`date $$DFLAG '+%d %h %Y'`; \
 	echo "Stuffing version number ${REL} into $@"; \
 	rm -f $@; \
 	sed \

mkhelp.py

@@ -2,8 +2,10 @@
 
 import time
 import sys
+import os
 
-time = time.gmtime()
+epoch = os.environ.get("SOURCE_DATE_EPOCH")
+time = time.gmtime(None if epoch == None else int(epoch))
 print("/* This file was generated by mkhelp.py from less.hlp at "\
     "%d:%02d GMT on %d/%d/%d */\n" %
     (time.tm_hour, time.tm_min, time.tm_year, time.tm_mon, time.tm_mday))