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>