Translations: Add custom .pot header with SPDX license identifier.

The same is used for both po/xz.pot and po4a/xz-man.pot.

Makefile.am

@@ -50,6 +50,7 @@ uninstall-local:
 endif
 
 EXTRA_DIST = \
+	po/xz.pot-header \
 	po4a \
 	extra \
 	doc/xz-logo.png \

po/xz.pot-header

@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: 0BSD
+#
+# SOME DESCRIPTIVE TITLE.
+# Copyright (C) The XZ Utils authors and contributors
+# This file is distributed under the BSD Zero Clause License.
+# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.
+#

po4a/update-po

@@ -41,3 +41,11 @@ po4a --force --verbose \
 	--package-version="$PACKAGE_VERSION" \
 	--copyright-holder="The XZ Utils authors and contributors" \
 	po4a.conf
+
+# Add the customized POT header which contains the SPDX license
+# identifier and spells out the license name instead of saying
+# "the same license as the XZ Utils package".
+mv xz-man.pot xz-man.pot.tmp
+cat ../po/xz.pot-header > xz-man.pot
+sed '1,/^#$/d' xz-man.pot.tmp >> xz-man.pot
+rm xz-man.pot.tmp