build: make it even more reproducible

* Makefile.am (today): Consistently print the UTC date in C locale.
* configure.ac (RPM_CHANGELOGTIME, DEB_CHANGELOGTIME): Likewise.
* maint/update_copyright_years.sh: Likewise.
* copyright-year-gen: Likewise.  When $SOURCE_DATE_EPOCH is non-empty,
use it as fallback date before the current system date.

References: https://github.com/strace/strace/pull/68

Makefile.am

@@ -992,7 +992,7 @@ dist-hook:
 	${AM_V_GEN}echo $(COPYRIGHT_YEAR) > $(distdir)/.year
 	${AM_V_GEN}echo $(MANPAGE_DATE) > $(distdir)/.strace.1.in.date
 
-today = $(shell date +%Y-%m-%d)
+today = $(shell LC_TIME=C date -u +%Y-%m-%d)
 version_regexp = $(subst .,\.,$(VERSION))
 news_check_regexp = 'Noteworthy changes in release $(version_regexp) ($(today))'
 

configure.ac

@@ -46,9 +46,9 @@ AM_INIT_AUTOMAKE([foreign nostdinc dist-xz silent-rules parallel-tests 1.13])
 AM_MAINTAINER_MODE
 AC_CANONICAL_HOST
 
-RPM_CHANGELOGTIME="$(LC_TIME=C date '+%a %b %d %Y')"
+RPM_CHANGELOGTIME="$(LC_TIME=C date -u '+%a %b %d %Y')"
 AC_SUBST(RPM_CHANGELOGTIME)
-DEB_CHANGELOGTIME="$(LC_TIME=C date -R)"
+DEB_CHANGELOGTIME="$(LC_TIME=C date -u -R)"
 AC_SUBST(DEB_CHANGELOGTIME)
 
 AC_PROG_CC

copyright-year-gen

@@ -2,6 +2,7 @@
 
 : ${YEAR_FILE:=$1}
 : ${DEFAULT_YEAR:=$2}
+LC_TIME=C; export LC_TIME
 
 year=
 
@@ -13,11 +14,17 @@ year=
 [ -f "${YEAR_FILE}" ] && year="$(cat "${YEAR_FILE}")"
 
 [ -n "${year}" ] ||
-	year="$(date +%Y -d "$(git show --format=format:%cD --no-patch)")"
+	year="$(date -u +%Y -d "$(git show --format=format:%cD --no-patch)")"
 
-[ -n "${year}" ] || year="${DEFAULT_YEAR}"
+[ -n "${year}" ] ||
+	year="${DEFAULT_YEAR}"
 
-[ -n "${year}" ] || year="$(date "+%Y")"
+[ -n "${year}" ] ||
+	[ -z "${SOURCE_DATE_EPOCH-}" ] ||
+	year="$(date -u +%Y -d "@${SOURCE_DATE_EPOCH}")"
+
+[ -n "${year}" ] ||
+	year="$(date -u +%Y)"
 
 [ -n "${year}" ] || {
 	echo >&2 'Undefined year.'

maint/update_copyright_years.sh

@@ -40,6 +40,7 @@ Headers updated automatically with
 : ${CALL_GIT_ADD=0}
 : ${CALL_GIT_COMMIT=0}
 : ${GIT_COMMIT_TEMPLATE=$DEFAULT_GIT_COMMIT_TEMPLATE}
+LC_TIME=C; export LC_TIME
 
 # These files are only imported into strace and not changed.
 # Remove them from the list once they have been changed.
@@ -121,9 +122,9 @@ process_file()
 		continue
 	fi
 
-	last_commit_year=$(date +%Y -d "$(git log -n1 --format=format:%aD \
+	last_commit_year=$(date -u +%Y -d "$(git log -n1 --format=format:%aD \
 		-- "$f")")
-	first_commit_year=$(date +%Y -d "$(git log --reverse --format=format:%aD \
+	first_commit_year=$(date -u +%Y -d "$(git log --reverse --format=format:%aD \
 		-- "$f" | head -n 1)")
 	copyright_year=$(printf '%s' "$copyright_year_raw" |
 		sort -r -n | head -n 1)