add original C links to entry navigation bars

The navigation bar for a winning entry now has a "_Original C_" link
(between the ✅ and the Inventory on the right hand side) so that one
may easily view the original C code that was submitted for IOCCC judging.

Ran `make www timestamp` to perform the above.

bin/subst.entry-index.sh

@@ -136,7 +136,7 @@ export LC_ALL="C"
 
 # set variables referenced in the usage message
 #
-export VERSION="2.0.2 2025-05-18"
+export VERSION="2.1.0 2025-08-10"
 NAME=$(basename "$0")
 export NAME
 export V_FLAG=0
@@ -756,6 +756,17 @@ echo "PROG_TEXT=C code"
 echo "-s"
 echo "MAKEFILE_LINK=$REPO_URL/$YYYY_DIR/Makefile"
 echo "-s"
+if [[ -f $YYYY_DIR/$ENTRY_DIR.orig.c ]]; then
+    echo "ORIG_LINK=$REPO_URL/$YYYY_DIR/$ENTRY_DIR.orig.c"
+elif [[ -f $YYYY_DIR/prog.orig.c ]]; then
+    echo "ORIG_LINK=$REPO_URL/$YYYY_DIR/prog.orig.c"
+else
+    echo "$0: ERROR: cannot determine original source code file for: $YYYY_DIR" 1>&2
+    exit 7
+fi
+echo "-s"
+echo "ORIG_TEXT=Original C"
+echo "-s"
 echo "MAKEFILE_TEXT=Makefile"
 echo "-s"
 echo "INVENTORY_LINK=#inventory"