Drop the date from the committed man page (#3659)

The date in the .TH line of jq.1.prebuilt changed whenever the manual was
regenerated in a new month, causing merge conflicts between pull requests
that touch the manual, and it did not reflect the release date anyway.
Generate the committed man page without a date.

Stamp the date into jq.1 at build time instead, taking it from the current
commit, or from SOURCE_DATE_EPOCH where that is set, so that release
tarballs and distribution builds still carry one and the behaviour added
in #3514 is kept. Write it in UTC and in ISO 8601 rather than the previous
"%B %Y", for which mandoc reports "cannot parse date, using it verbatim".

Makefile.am

@@ -190,8 +190,18 @@ else
 	@echo As a result, the manpage is out of date.
 endif
 
+# The committed manpage has no date to avoid merge conflicts. Stamp the date
+# of the last commit when building in a git repository, including on make
+# dist, so that release tarballs ship a dated jq.1. Respect SOURCE_DATE_EPOCH
+# so that distribution builds can pin the date.
 jq.1: jq.1.prebuilt
-	$(AM_V_GEN) cp $(srcdir)/jq.1.prebuilt $@
+	$(AM_V_GEN) date=`if test -n "$$SOURCE_DATE_EPOCH"; then \
+	    date -u -d "@$$SOURCE_DATE_EPOCH" +%F 2>/dev/null || \
+	    date -u -r "$$SOURCE_DATE_EPOCH" +%F 2>/dev/null; \
+	  elif test -e $(srcdir)/.git; then \
+	    cd $(srcdir) && git log -1 --format=%cd --date=short 2>/dev/null; \
+	  fi`; \
+	sed 's/^\.TH "JQ" "1" ""/.TH "JQ" "1" "'"$$date"'"/' $(srcdir)/jq.1.prebuilt > $@
 
 CLEANFILES += jq.1