docs-rst: enable the Sphinx math extension

This extension will be used by the media books.

The name of the math image extension changed on Sphinx 1.4.x,
according with:
	http://www.sphinx-doc.org/en/stable/ext/math.html#module-sphinx.ext.imgmath

Let's autodetect, to keep building with versions < 1.4.
Suggested-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/conf.py

@@ -14,6 +14,11 @@
 
 import sys
 import os
+import sphinx
+
+# Get Sphinx version
+major, minor, patch = map(int, sphinx.__version__.split("."))
+
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
@@ -31,6 +36,12 @@ from load_config import loadConfig
 # ones.
 extensions = ['kernel-doc', 'rstFlatTable', 'kernel_include']
 
+# The name of the math extension changed on Sphinx 1.4
+if minor > 3:
+    extensions.append("sphinx.ext.imgmath")
+else:
+    extensions.append("sphinx.ext.pngmath")
+
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']