Add missing build aux file for obtaining the git version.

This is needed to make commit e8e7139 work for people that are not me.

build-aux/git_version.py

@@ -0,0 +1,16 @@
+#!/usr/bin/env python3
+
+import re
+import sys
+import subprocess
+
+try:
+  version = subprocess.check_output(
+    ['git', 'describe', '--all', '--long', '--dirty'],
+    stderr=subprocess.DEVNULL
+  )
+except:
+  sys.exit(1)
+
+version = str(version, encoding='UTF-8').strip()
+print(re.sub(r'^heads\/(.*)-0-(g.*)$', r'\1~\2', version))