groff: use changelog date rather than file modification date in man pages

The source checkout date is not particularly relevant, and this makes
groff man pages build reproducibly.

Reviewed by:	bapt
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D8158

gnu/usr.bin/groff/Makefile.inc

@@ -119,7 +119,7 @@ revision=`sed -e 's/^0$$//' -e 's/^[1-9].*$$/.&/' $(GROFF_DIST)/REVISION`
 	-e "s;@TMAC_MDIR@;$(tmacdir)/mm;g" \
 	-e "s;@BROKEN_SPOOLER_FLAGS@;$(BROKEN_SPOOLER_FLAGS);g" \
 	-e "s;@VERSION@;$(version)$(revision);g" \
-	-e "s;@MDATE@;`$(SHELL) ${GROFF_DIST}/mdate.sh $<`;g" \
+	-e "s;@MDATE@;$(MDATE);g" \
 	-e "s;@g@;$(g);g" \
 	-e "s;@G@;`echo $(g) | LC_ALL=C tr a-z A-Z`;g" \
 	$< >$@
@@ -141,4 +141,5 @@ TOPREL?=	..
 GROFF_DIST=	${.CURDIR}/${TOPREL}/../../../contrib/groff
 DIST_SUBDIR?=	${.CURDIR:T}
 DIST_DIR=	${GROFF_DIST}/${DIST_SUBDIR}
+MDATE!=		sh ${.CURDIR}/${TOPREL}/mdate.sh ${GROFF_DIST}/ChangeLog
 .PATH: ${DIST_DIR}

gnu/usr.bin/groff/mdate.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+# $FreeBSD$
+
+set -e
+test -r "$1"
+export LC_ALL=C
+changelog_date=$(sed -E -n 's/^([0-9]{4}-[0-9]{2}-[0-9]{2}).*$/\1/p' "$1" |\
+    head -n 1)
+echo $(date -j -f %Y-%m-%d +"%e %B %Y" $changelog_date)