Add tools/vergen

Taken from GNU ease.js.

* tools/vergen: Added script.

tools/vergen

@@ -0,0 +1,37 @@
+#!/bin/sh
+# Generates version string
+#
+#  Copyright (C) 2012, 2013, 2014 Free Software Foundation, Inc.
+#
+#  This file is part of GNU ease.js.
+#
+#  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/>.
+##
+
+# for distributions
+if [ -s version ]; then
+  cat version
+  exit
+fi
+
+git rev-parse --show-toplevel >/dev/null 2>&1 || {
+  echo 'fatal: git repository is not available'
+}
+
+version="$( git describe --match='[0-9]\.*' --abbrev=7 HEAD 2>/dev/null )"
+git update-index -q --refresh
+test -z "$( git diff-index --name-only HEAD)" || version="$version-dirty"
+
+echo -n "$version"
+