Don't set html_last_updated_fmt without git

Using a timestamp as value for "html_last_updated_fmt" isn't good
for the reproducible builds initiative from different downstream
distributions because the build result always differs due to the
timestamp in the documentation.
Also when not setting "html_last_updated_fmt", the value can be set
via command line when running "sphinx-build -D".

Change-Id: Ic8912e95e8e298fede1d94c20d3167d1f206df1e

doc/source/conf.py

@@ -4,7 +4,7 @@ import datetime
 import os
 import subprocess
 import sys
-import time
+import warnings
 
 sys.path.insert(0, os.path.abspath('../..'))
 # -- General configuration ----------------------------------------------------
@@ -78,7 +78,8 @@ try:
     html_last_updated_fmt = subprocess.Popen(
         git_cmd, stdout=subprocess.PIPE).communicate()[0]
 except Exception:
-    html_last_updated_fmt = time.ctime()
+    warnings.warn('Cannot get last updated time from git repository. '
+                  'Not setting "html_last_updated_fmt".')
 
 # Grouping the document tree into LaTeX files. List of tuples
 # (source start file, target name, title, author, documentclass