build: support creating reproducible tarball contents

We already support reproducible builds since commit v8.24-99-gc1b3d6587,
and this adjusts that change to also support reproducible
tarball contents with subsequent runs of `make dist`.

* Makefile.am: Don't create a varying .timestamp file, instead ...
* man/local.mk: Rely on the timestamp of the .tarball-version file.
Fixes https://bugs.gnu.org/72232

Makefile.am

@@ -82,14 +82,12 @@ BUILT_SOURCES = .version
 
 # Have no read-only files in the tarball to allow easy removal.
 # Have .tarball-version based versions only in tarball builds.
-# Have .timestamp based dates only in tarball builds.
 # The perl substitution is to change some key uses of "rm" to "/bin/rm".
 # See the rm_subst comment for details.
 # The touch avoids a subtle, spurious "make distcheck" failure.
 dist-hook: gen-ChangeLog
 	$(AM_V_GEN)chmod -R +rw $(distdir)
 	$(AM_V_GEN)echo $(VERSION) > $(distdir)/.tarball-version
-	$(AM_V_GEN)date +%s > $(distdir)/.timestamp
 	$(AM_V_at)perl -pi -e '$(rm_subst)' $(distdir)/Makefile.in
 	$(AM_V_at)touch $(distdir)/doc/constants.texi \
 	  $(distdir)/doc/coreutils.info

man/local.mk

@@ -194,7 +194,8 @@ endif
 	  && $(MKDIR_P) $$t						\
 	  && (cd $$t && $(LN_S) '$(abs_top_builddir)/src/'$$prog$(EXEEXT) \
 				$$argv$(EXEEXT))			\
-	&& : $${SOURCE_DATE_EPOCH=`cat $(srcdir)/.timestamp 2>/dev/null || :`} \
+	&& : $${SOURCE_DATE_EPOCH=`date -r $(srcdir)/.tarball-version +%s \
+				   2>/dev/null || :`} \
 	&& : $${TZ=UTC0} && export TZ					\
 	&& export SOURCE_DATE_EPOCH && $(run_help2man)			\
 		     --source='$(PACKAGE_STRING)'			\