Remove sha1sum dependency for doc generation

On MacOS X there is no sha1sum. So to simplify doc generation on those systems
use a Python script instead to generate a unique string from the repository
path.

doc/utils/sha1sum.py

@@ -0,0 +1,7 @@
+#!/bin/env python
+# simple utility which reimplements sha1sum using Python
+import hashlib
+import sys
+s = hashlib.sha1()
+s.update(sys.stdin.read().encode())
+print(s.hexdigest())