libxfce4ui: Change vendor info

debian/rules passes value of XFVENDOR to the package's configure script
as the value of the "--with-vendor-info" option.  This sets the string
in the "distributed by" message in the xfce4-about program.

debian/vendorinfo also needs to be changed because debian/rules installs
this file to /usr/share/xfce4/vendorinfo if the value of XFVENDOR is not
"Xubuntu".

Fixes issue #15432.

helpers/make-libxfce4ui

+#!/bin/sh
+#
+#    Copyright (C) 2015  Patrick "P. J." McDermott <pj@pehjota.net>
+#
+#    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 <http://www.gnu.org/licenses/>.
+
+VERSION=1
+
+. ./config
+
+sed -i 's/XFVENDOR=.*/XFVENDOR=Trisquel/' debian/rules
+
+cat >debian/vendorinfo <<-'EOF'
+	Distributed by Trisquel GNU/Linux developers:
+	<mailto:trisquel-devel@listas.trisquel.info>
+
+	Bugs should be reported to Trisquel's issue tracker:
+	<http://trisquel.info/en/project/issues/trisquel>
+	EOF
+
+changelog "Rebranded for Trisquel"
+
+compile