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+# ===========================================================================
+# https://www.gnu.org/software/autoconf-archive/ax_build_date_epoch.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+# AX_BUILD_DATE_EPOCH(VARIABLE[, FORMAT[, ACTION-IF-FAIL]])
+#
+# DESCRIPTION
+#
+# Sets VARIABLE to a string representing the current time. It is
+# formatted according to FORMAT if specified, otherwise it is formatted as
+# the number of seconds (excluding leap seconds) since the UNIX epoch (01
+# Jan 1970 00:00:00 UTC).
+#
+# If the SOURCE_DATE_EPOCH environment variable is set, it uses the value
+# of that variable instead of the current time. See
+# https://reproducible-builds.org/specs/source-date-epoch). If
+# SOURCE_DATE_EPOCH is set but cannot be properly interpreted as a UNIX
+# timestamp, then execute ACTION-IF-FAIL if specified, otherwise error.
+#
+# VARIABLE is AC_SUBST-ed.
+#
+# LICENSE
+#
+# Copyright (c) 2016 Eric Bavier <bavier@member.fsf.org>
+#
+# This program is free software: you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by the
+# Free Software Foundation, either version 3 of the License, or (at your
+# option) any later version.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+# Public License for more details.
+#
+# You should have received a copy of the GNU General Public License along
+# with this program. If not, see <https://www.gnu.org/licenses/>.
+#
+# As a special exception, the respective Autoconf Macro's copyright owner
+# gives unlimited permission to copy, distribute and modify the configure
+# scripts that are the output of Autoconf when processing the Macro. You
+# need not follow the terms of the GNU General Public License when using
+# or distributing such scripts, even though portions of the text of the
+# Macro appear in them. The GNU General Public License (GPL) does govern
+# all other use of the material that constitutes the Autoconf Macro.
+#
+# This special exception to the GPL applies to versions of the Autoconf
+# Macro released by the Autoconf Archive. When you make and distribute a
+# modified version of the Autoconf Macro, you may extend this special
+# exception to the GPL to apply to your modified version as well.
+
+#serial 2
+
+AC_DEFUN([AX_BUILD_DATE_EPOCH],
+[dnl
+AC_MSG_CHECKING([for build time])
+ax_date_fmt="m4_default($2,%s)"
+AS_IF([test x"$SOURCE_DATE_EPOCH" = x],
+ [$1=`date "+$ax_date_fmt"`],
+ [ax_build_date=`date -u -d "@$SOURCE_DATE_EPOCH" "+$ax_date_fmt" 2>/dev/null \
+ || date -u -r "$SOURCE_DATE_EPOCH" "+$ax_date_fmt" 2>/dev/null`
+ AS_IF([test x"$ax_build_date" = x],
+ [m4_ifval([$3],
+ [$3],
+ [AC_MSG_ERROR([malformed SOURCE_DATE_EPOCH])])],
+ [$1=$ax_build_date])])
+AC_MSG_RESULT([$$1])
+])dnl AX_BUILD_DATE_EPOCH